Dystopia: Humanity cannot give itself a purpose directly, it's 'human nature' I assume. Truthfully, I think that, unconsciously, we do what we do out of desperation for survival.
Thomas hobbes Once wrote 'When all the world is overcharged with inhabitants, then the last remedy of all is war, which provideth for every man, by victory or death'. there are many interpretations of this, but my interpretation is that, if humanity is not at conflict with itself, that we'd 'overcharge' our planet, and kill it. we'd then die off.
It's just a theory, but history seems to support it.