One thing you need to understand is that, for the Spanish and English in the colonial era, the "three Gs" (God, glory and gold) were noble causes. The notions of the "divine right of kings" and the continuing Great Commission of Christianity painted these men as not one iota less than the missionaries of God, charged by His Anointed Kings with spreading the Good News to all who would listen (and eradicating the unfaithful so that the faith may flourish), obtaining the preordained dominion of men over the land given to them, and glorifying God and His Anointed Kings with tribute of material wealth as the fruits of these missions.
So, you always have to take a subjective quality like "noble" in the contemporary context to the use of such a term. The expansion of religion, the conquering of land and indigenous peoples, and the exploitation of nature's bounty of material wealth were absolutely noble reasons in the 1500s for the actions of the conquistadors and the English colonialists.
Today, much of humanity has a very different set of principles, and a similarly different view of societies other than our own. Modern historians have heavily "de-romanticized" the notions of European exploration and colonization of the Americas, now aware of the true horrors visited on the indigenous tribes of the Americas by the Europeans. The "conquest of the Americas was seen as exactly that - an invasion of the New World - and the slaughter of the millions of native people living here is quite rightly seen as the textbook definition of genocide.
The fact that this term wasn't coined until 1944, to describe the actions of the Axis powers in Europe, is downplayed by modern scholars, as is the fact that the act of killing every man, woman and child that you view as your enemy was considered by all sides as a totally valid tactic to win a war. Keeping noncombatants in your newly conquered territory alive and productive under your control was a purely economic decision until about the 1800s, with the Hague Declaration being a very monumental piece of international law in that, for basically the first time in human history, there were "rules" of war both sides were bound to follow.
These contextual semantics are downplayed, because our "moral calculus" has evolved over time. The worth of a human life has increased in the general case, and as such we tend toward the deontological over the relativist when looking at actions resulting in human death, especially a lot of it. We retroactively condemn these actions as criminal, and that's not wrong, but that's not how it was viewed at the time or indeed for decades or even centuries thereafter. In the interest of evolving the collective human conscience beyond such barbarism, we teach these events as examples of what not to do (the whole "those who don't study history are doomed to repeat it"), but IMHO the omission in many studies of the contemporary context and worldview of those involved make it that much more likely that similar atrocities will be committed, because while we will know what others did, the subtleties of how and why things got to that point will have been lost, leaving us that much less able to realize what we are about to do until "too many wheels are turning".
Anyway, the connotation behind the concept of a "higher purpose" has also changed dramatically in the 20th and 21st Centuries. Modern human society is much more secular than it was even 100 years ago, because we've found that a zealous belief that you are right because your religion says so has been a primary cause of human death for most of human history. Today, explorers "come in peace for all mankind", even though they still leave the flag of their home nation wherever they've been. The purpose is to be able to say, for yourself, for your country and for humanity, "I/we did that". You pushed the boundaries of human experience or ability, by being the first to do something, thus writing yourself into the history books of humanity forever.
Who knows; in another 200 years or so, we might look back on such "self-centered drives to accomplish" as being totally against the spirit of humanity as we understand it in the 2200s. While we like to think that what we do, we do for everyone, let's face it, having your name remembered forever by your posterity for what you did is a powerful drive. We might, in another 200 years, shift our moral calculus again toward the truly selfless, and such "selfish" acts as Neil Armstrong's will be deromanticized as thoroughly as John Smith's have been today.