As a non believer I often wonder about this.
Many people have put quite sensible answers but what I think is that all those answers are linked by a common denominator, that is to transmit as much of their DNA to the next generation as possible. Making money certainly produces that effect but also making life better for others because you'll be loved by others and they'll help you to survive. Everything converges on leaving copies of our DNA and because there are many ways of doing that, some of them in absolute opposition to one another, then that's why there are so many different answers. What I don't understand is the many people who wrote down No Purpose at all. However even if they think so on a practical level they behave in the best way for them to maximise their chances to leave copies of their DNA.
For everyone it's different, but for me, I think it's to experience as much good as I can in this world. Only by doing things and achieving goals do you grow as a person and I think that what really makes people happy is the good experiences in life.
I thought I was going to be original here, but it seems as though everyone beat me to the punchline. The purpose of my life is finding that which makes me a better person, more educated person, and a person who understands that finding out I'm wrong about something is equally as valuable as finding out I'm correct about something. (If that doesn't make sense I can explain later.) My purpose is for me to discover and never to impose any part of it on anyone else.