I'm frequently seeing this presumption everywhere that there is an overarching purpose to life, and it often comes in a question form from theists directed at atheists. They'll ask something like "What's the purpose of life without god?", which I find incredibly naive and condescending. Firstly, I'm not convinced that god exists, so I don't see how it (god) could possibly give life purpose. I think that by 'purpose' they must mean 'subservience to an invented deity with the childish hope of an afterlife'.
On a different note, I think it is possible to build a fairly strong case against a blanket purpose for life. When one considers the fact of evolution, it is quite obvious that life as we know it, is a mere product of chance and accident, we were formed by an unguided process. How illogical then is it to try and impose a generic purpose to all of life? Let's take another product of chance, an accident of the universe, an asteroid, meteorite, comet etc. Is there a transcendent purpose for their existence? I think not, so likewise it is ignorant to suggest that there is a purpose to be discovered for life.
Now before you make any assumptions, I'm not a pessimistic nihilist. We have brains, and from our direct observations, our race is the highest form of intelligence in this region of the universe, so why should we require an unseen higher power to designate us a purpose in life? From my perspective, it makes much more sense to create purpose for ourselves, rather than to wait for purpose to come to us. If you look at all of the great figures in history, every single one of them made a conscious effort to achieve their goals, some may have just been in the right place at the right time, but the majority of them didn't sit there waiting for their god to assign them their lot. The only purpose in your life is the one you create for yourself, you're the only one that can be held responsible for your successes.
So there, I think that just about sums up what I think about the purpose of life.