Yes, I appreciate the noble sentiments about morality (yes, you don't need god(s) to act morally) and posthumous legacies (despite those you leave behind also facing oblivion). But while I too abide by these life philosophies (which I find baffling), the simple fact is that ultimately they don't matter. People keep saying "it matters" but how does it matter exactly? Whether we as individuals or as a society choose to nurture each other through life as painlessly as is humanly possible (Earth as giant hospice), or relish tormenting the weak, what difference is all that going to make when there's nobody left?
One day the Earth will die and then eventually the universe, so any and all great gestures and noble causes will have been for nought. Too melodramatic, you say? When I was in the 5th grade, after some kind of biology lesson, a fellow student came up to me and said, "I'd hate to be a butterfly; they don't live very long". I replied, "One year or eighty, how will you know the difference when you're dead?" He looked at me like I had some kind of illness.
The question is, armed with this knowledge, why do we care about the wellbeing of ourselves and others when one day it will be as though we had never existed at all. One day the passing on of legacies will cease. Perhaps self-awareness comes packaged with an unconscious yet powerful optimism, where we want to stick around and get on with life in the hope that something will save us from oblivion. I guess we have to believe that -- what other choice do we have?