Assuming that interstellar travel is possible but quite slow (slower than light) and expensive (in energy and resources) and that faster than light communication is not possible, what could motivate a sentient species to try to conquer its home galaxy, by mean of self-replicating von Neumann machines or generation spaceships (rather than investing these resources in its home system).
closed as off-topic by Mormacil, Azuaron, Frostfyre, sphennings, JDługosz May 9 '17 at 13:52
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If it is technologically and economically possible to conquer the galaxy, someone will do it, even if it is just by accident of free running Von Neumann mechs. So: why conquer the galaxy? So that you aren't ruled by or killed off by someone else. It is the same reason corporations, cities, and nations tend to grow until all available resources are claimed, and then they pause for a while while they use their internal resources. Then they go to war when one of them runs out of resources and cannot trade for more (or slightly before they run out).
Existence is a zero-sum game at every scale. We have a lot of good ideas about sharing resources equally, but that just means that all parties will sunset at the same time. If one party wants to outlive the others, they have to pull resources somehow. If the others aren't feeling self-sacrificial... war.
Rare and scattered resources. The rare element Unobtanium-5 is only found in certain star systems controlled by another faction, while your vast stores of Unobtanium-2 are guarded closely and coveted by many.
Ideology. Some races may feel it's their destiny, others may be paranoid or xenophobic.
Greed They payoff is eventually going to be worth it. If you build into the game mechanics, a multiplicative effect for owning more star systems, such as increased diplomatic power in trade negotiations and/or production, this would motivate them.
In addition to Richard U and Kilisi's answers, there are some other possibilities:
- To combat population boredom/restlessness (post-scarcity society)
- Extreme population growth (extremely high birth rate with low mortality rate and long lifespan)
- As a political move to create artificial scarcity and maintain control over a population (Orwell's 1984 style)
I'm sure there are others, but those are all viable.
Go the Rick & Morty route, have an omnipotent force from outside the galaxy impose rules upon those within it, for their own entertainment. It's already a game, why not game-ify the world of those within? If you can't think of a valid justification for your game logic, invoke a higher power.
Show me what you got!