My theory is that there is no higher or over-arching purpose to this existence. Now, before I get called a nihilist, I'd like to say that I actually find this to be a reason for hope rather than despair. Simply because, if there is no grand purpose, no ultimate reward or final punishment for our actions, then what we do is all that matters. So the only real way to go is to do what you think right and see how it works out in the end.

By do what you think is right, obviously, I don't think things like genocide/rape/child-abuse/etc are a good idea and should probably be avoided even if you do think they're right, but the lack of that higher purpose makes the deeds we do on earth all the more important.