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Setting:
In my world the cellular aging process of humans is stopped and life is nearly endless. Humanity is spreading into space to colonize planets, so living place and resources are not big problems.
Question:
In such circumstances would there be enough recruits for military services? Why would one risk to die in a war instead to live an infinite long life?
This is a hard question to give a definitive answer to. So I'm going to point out some pieces that may help you examine it. Ultimately, there are some pieces that remain unanswered in your setting that may impact the answer. I will try to address these.
Please note that this question invites biased answers, because it is a question of human nature, and philosophers have argued about these topics since the dawn of recorded history, and still haven't come to anything like an agreement. I apologize in advance for any non-objective input in this answer.
First off, a thing that wasn't clear in your setting...
Wealth distribution.
In short, if there are still rich and poor, then with an unlimited life, the rich will get richer...and the poor may remain poor their entire lives. If you are barely eking out an existence on the edges of society...it may be worth risking your life rather than spending eternity barely surviving.
Existence of Tyrants
These have existed since...pretty much always. You get someone who says 'obey me with a risk of death, or face immediate death.' Well, he has conscripts to fight now. Then others, if they want to stop him, must build armies as well. After all, you don't get a war if only one side is ready to fight. They generally call that a massacre, not a war.
Boredom
Eventually, if you live forever, you are going to run out of crap to do. You only have one risk that is a real thing. Death. Anything else...well, you can recover from it in time. So the only real thrill is something with the risk of death.
Human Nature
This is the piece that, as I warned, shows my bias about human nature. If you disagree with me, then for the purposes of your story, ignore this block. I don't believe that people are intrinsically good. There will always be those who want more than they have...and there will always be those that don't care who they have to hurt to get it. That is, after all, a fair definition of a Psychopath. Also, I believe that there will always be ideological differences that people are willing to fight for. And, if you remember, old people tend to be even more set in their beliefs than someone younger, and can be rather belligerent in defending them. Add in the human-nature 'Us Versus Them' mentality (want to demonstrate it? Look at sports team fans and the fans of their rival team), add in a few thousand years to let it all steep...and you have a recipe for some very deep seated hatred.
So, in conclusion...I don't think that agelessness would mean the end of war.
Why would anyone go to war now?
You named a couple reasons: resources and space, but those aren't really very good reasons. Both are sort of "get a better life for descendants", though rebuilding is a better way to do that.
Freedom - This one is pretty good, and also fits under "get a better life for descendants". Get out from under some tyrannical rule, be able to make your own decisions and live the life you want and not have your future decided by someone else...
Money - Soldier of fortune, mercenary, whatever. You have a skill, someone else will pay you to use it.
Power - Get the other guy to do what you want. Kind of the opposite of freedom.
Glory - Everyone wants to look good, be a hero. In a world without war, how do you become a hero and set yourself apart from everyone else?
Adventure - Get away from your everyday life and have new (horrible) experiences.
The key to most of these is the fact that old people send young people to war, and young people sign up because they don't know any better and think they are immortal already.
If immortality was real for everyone, hopefully people would value their lives more, but it really depends on what the 'enemy' is, and whether it's worth it.
If they are defending on the other hand, you'll most likely get lots of volunteers.
I think a bigger thing is that without FTL, inter-planetary war is pretty much pointless. We're not going to launch ships to attack Alpha-Centauri when it's going to take a thousand years to get there and it's your great great grand children that will do the actual fighting.
There'd certainly be wars over food. Imagine the population growth, the huge boom that would happen if there were no natural deaths. We'd quickly run into famine, and people would be dying in the streets. This would lead to factions competing for the increasingly scarce resources of food, land to grow it, water to feed the crops, and fertilizer for the crops. Then wars would be fought over the resources to defend that land (oil/coal/natural gas, uranium, metals). Possibly wars fought to obtain slave labor, then slave uprisings.
Boredom, mostly.
After a certain amount of time, people get malaise. Everything seems old and "already done." There are only so many sunsets you can watch before it loses the emotional power. People are always willing to sacrifice for something that feels broader and more important than themselves.
Then there's the fact that even when we're immortal, we're still human beings who are prone to all sorts of emotional imbalances. People can be angry, bloodthirsty, psychopathic... You'd much rather some of those people be soldiers than wreaking havok in the general population.
It's likely that you'd experience a lull in recruiting of a couple hundred years as the first immortals lived their lives to the fullest, but eventually your recruitment would fill with gods weary of this mortal shell and eager to experience the unparalleled thrill of death.
What if people fought wars AGAINST immortality? They specifically saw that the immortal life had some severe unintended consequences for people, culture, and society (why get anything done when you can procrastinate literally forever?) and knew that the only way to save our humanity was to destroy immortality?
So you have the "perfect immortals" being fought by those demanding a mortal imperfect life as the purer human experience...
If you could no longer die of old age, I suspect the main changes in our culture would be the following:
However, just because human life would be considered more precious than now doesn't mean necessarily that you wouldn't see people forced into military service for war. You could argue that even now, there shouldn't be war, because if you get shipped off to battle, you are risking your life. People do so for their country and because they think it is the right thing to do.
Though, I suspect that even so, if you could live forever, you would not throw yourself in a situation willingly where you would likely die. As such, I imagine that war would be fought heavily with drones and remotely piloted aircraft. In a typical war, there would likely be very few casualties. The actual act of destroying many people would be considered an atrocity, killing people who may have been alive for centuries. As such, even strategic targets would likely be mostly be manufacturing plants of these drones, which themselves would be run heavily by robots. To attack a city would be cause to warrant the brunt of the world's armies against you, and your allies to have difficulty justifying their cause.
However it should also be said that it is an atrocity to the general public. To politicians and leaders, this is an excuse for war, however they would not have any difficulty putting the "younger" people in percarious positions. If you think such things aren't possible, consider that many drafted and sent to Vietnam during the Vietnam war were young and in a lower poverty bracket.
As to whether or not the politicians themselves would have motivation to go to war, well, so long as they are not the ones fighting, their motivation would be no different than today. Technology has proven time and time again not to be a cause for peace, but rather an instrument of war.
To quote Albert Einstein:
I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones.
Your second point is more likely to end war than the first - assuming first that 'expanding into space' also means 'finding infinite resources in space for human life', since otherwise, you're going to have the same problems you'd have here on Earth, but on a wider scale.
What you have to remember about war is that, historically, it was waged because X group wanted the resources of Y group, and was willing to take them by force. And the chief maturation of war past the tribal state was that war was fought not for food or shelter, but for land ownership and space to expand a growing and restless population. Without a life cycle, that population would grow more and more restless with each passing generation (exponentially, assuming every human being also remains fertile, and our urge to procreate does not expire).
Assuming humanity did reach a point where space travel was not only easy for a mass-market, but also viable as a way to expand into new living arrangements, that would solve the problem that your first point creates. But with any type of limited resource, be it food, shelter, luxuries, or even just personal space, the greater the number of people in a more densely-packed environment, the more likely a conflict is to erupt.
Guildbounty makes very good points, I second all of them. War is, sadly, a fact of nature. I don't think people go to war now thinking "okay, I'm only going to have 60 years to live of my natural life, so I might as well risk myself at 18 now, but if I had 80 years, then I wouldn't take the risk! People go to war expecting to NOT die (sure they know the odds, but we all think were be the one to beat them), or they decided death is worth it, but they don't do a cost-reward analysis that considers expected years to live. After all traditionally young men, barely 18, went to war more often then the 30-40 year olds despite the older ones having fewer years left to live.
However, I mainly wanted to focus on a specific implication of ending old age which is relevant here. The problem with no death is that people start having children faster then old people die out. This leads to competition over resources. Many apocalyptic utopians have been generated from the idea that people stopped aging, since we now have no resources to provide for everyone and the only solution is to kill people.
Your situation is a little different since people can still die, just not of old age. Death from accidents, murder, disease etc will still remove population, perhaps we expect to live to 150 instead of 80 on average. However, this would mean that women are fertile throughout their lives, instead of just until 40, and so would still keep reproducing. This means we would be haivng more children even as those that are born live longer, and thus consume resources longer. More people who consume more resources is not sustainable, the resources on earth can not keep up with the constantly increasing demand.
This would could be made theoretically sustainable if you suggest extensive use of birth control and similar regulation was in place to keep birthrates roughly equal to death rates. Alternatively if you add space travel then it's possible to allow this so long as were able to colonize new worlds, with new resources, fast enough to keep up with constantly increasing demand. I suppose you could also claim better technology and infrastructure, combined with a significantly increased emphasis on sustainability and production of food due to the new demand, has allowed us to generate food better. We started building massive hydroponic gardens using genetically engineered food and renewable resources to drastically boost the rate we produce food, but that hand wave can only go so far.
However, if you don't add one of the above approaches you will have far more wars as people fight over increasingly limited resources. The motivation is so that country X will have enough food to maintain their population, but the deaths themselves are what keep us from overwhelming our society. This seems to lead to inevitable distopia, but you could mitigate it somewhat if you don't want a full distopia. If you imply a combination of the other approaches have been implemented to control our resource needs. Were using more significant birth control, but still birthing more then die of normal causes. We have gotten better at producing food thanks to both technology and granter emphasis on production, which means we can handle a larger population to an extent, and all of this means we can almost, but not quite, keep up with the rate humanity is growing. Thus wars occur semi-often over resources, but tend to be smaller skirmishes. Their enough to limit our population growth without full blown war occurring etc.
While it's not exactly a direct answer, there's a pretty good reference for this in the form of the Culture series by Iain M. Banks.
They're not, strictly speaking, immortal but close enough at 400 years. Even at that, the tech level implies that they could probably live longer if they actually wanted to. The AIs that run things, the Minds, are likely immortal by any useful definition.
The brutally applied logic is that once a society has reached a level where they can have anything they want, a subset of those people will be willing to risk their lives to protect that way of life, or to try to extend that way of life to others who haven't achieved that level of tech.
Extending the first of those reasons, I agree with the sentiments raised by other responders but feel that the answer can be simplified considerably and thus my answer is:
Regardless of technological level or life expectancy, there will always be armed conflict as long as there exists people who are willing to use violence to alter the status quo.
People will wish to change the status quo as long as alternatives can be imagined that will grant them a subjectively positive benefit.
The resources to raise an army is probably moot, the number of people willing to resist an aggressor tends to scale in proportion to the size and aggression faced.
I think that you could likely take any specific facet of a story and construct a set of solid reasons for groups fighting for and against that facet, so if you need to have a war within your story then all you have to do is find a suitable point of disagreement and build a divide out from that.
You only need to have a convincing reason for why that something is so important.
Volounteer armies are a luxury that happens only when the nation can get the required number of troops in this manner. In much of the world now, and in all of the world historically, your opinion won't matter if you'll be required to go to war. Declining conscription or draft generally is a serious criminal offense; and desertion on battlefield can be and has been punished by execution - soldiers can be given a choice to have a chance to die on the battlefield or die for sure immediately.
Furthermore, if a competing group has a large enough military (no matter how they obtain/enforce that) then your group has, in effect, a simple choice - be prepared to risk your life in a war, or surrender all rights that you may have to any group that is willing to fight and desires anything from you - property, freedom, and eventually life as well.
To bring Honor to your family. (Elders could pressure their younger relatives to perform military service to improve the prestige of the family as a whole)
To become Famous through your deeds if you manage to survive. Especially if all your feats are filmed and public like in "Running Man". Maybe scars are cool in the future.
Soldiers get Special Training which makes them eligible for jobs with high prestige (volunteer have no expectation of actually seeing combat, until...)
Blow stuff up. Explosives and weapons are highly regulated - except at combat academy.
Your question leaves open too many possibilities for there to be one answer. My answer in general would be that mostly there probably would not be wars, not because of lack of recruits, but because in order for humanity to survive its own 20th/21st Century abuses of its homeworld, and emerge as something cooperative and healthy enough to achieve colonization of distant worlds, defeating cellular aging and having no resource problems, then it probably has developed mindsets healthy enough that we aren't waging war on ourselves any more, and we see little reason to do so. If and when some of us revert or degrade to something hostile, or we face some external threat, then I am sure that there would be plenty of people willing to help and risk their ageless lives to do so (although we would probably have the technology to risk more robots than people at that point).
Why would someone risk losing their unaging life? Perhaps some wouldn't and others would, but I think that such a future culture would tend to be relatively wise and benevolent, and would have many people who would wish to serve to protect their friends, families, and this nice culture they have, and be willing to risk or in some cases even sacrifice themselves.
Not everyone cherishes their own survival above all other things, particularly after having experienced many decades of life.
Many people also believe that their physical body's life is not the same as their soul's life, and accept death as part of life, and just another step on their soul's path.
Potentially unlimited lifespan wouldn't change much.
Nobody joins the military planning to die for their country - you merely accept a certain level of risk in exchange for either the direct benefits of service or to try and make a better world to live in once the war is over.
Apart from the fact that eighteen year olds are not exactly known for risk aversion, the risk isn't really that great. You aren't using WWI tactics, so most soldiers survive, and it is quite possible to lose more soldiers to training accidents than to enemy action.
There are also situations where war is actually the less risky choice - think sitting safely at home until the invading space-nazis show up to execute you.
People choose to fight knowing that they might die. You may be coerced by conscription, but you can always at least try to flee the draft, go AWOL, or anything else that appears less risky than fighting, if you're so inclined after performing your moral and cost-benefit assessment. Perhaps "decide" would be better, but I'll stick with "choose".
If you die, you lose your whole life, however long that might otherwise have been. You might believe in an afterlife, or you might not. You might like your life or you might not. Your life might be under threat even if you don't fight, for example if your country has been invaded. You may have been propagandised, shamed into joining up, threatened with imprisonment or execution, or deceived as to the exact degree of risk. But if you fight, then you've chosen risk to life because not fighting is somehow worse.
Now, we might assume that people would prize a longer life more highly than a shorter one, but it doesn't follow that they would prize a life without ageing infinitely highly, such that they'd never risk it for any cause. Aside from anything else, not ageing doesn't imply infinite lifespan, since you will still in all probability have a nasty accident eventually.
So, it's entirely up to you whether there are "enough" recruits, depending how many your narrative requires and on how willing people are to risk their lives. There's no reason to assume that people would be absolutely unwilling to take risks of dying. However, as in the real world, you need to motivate the characters' choices if they choose war (politicians) or choose military service (soldiers).
All that said, the majority of real-life wars seem to have to do with living space and resources at some level. There are ideological causes for war as well, but if everybody is running quickly away from each other in different directions, then it's difficult to see what a war would be fought over. More realistically, even with rapid expansion into space some people would be running to the same place in competition, and therefore could come into conflict.
War, like everything in life, is a choice. People choose to fight for various reasons ... With immortality that choice doesn't change. As far as boredom goes - immortality doesn't change that either - it is driven by one's own personal desire -- for a better life for ourselves, our children - whatever. People are, in general, vagrants and leech off society; being immortal doesn't fix that. People have to want to have things be different -- regardless of other factors.
