My first post here, so hello everybody!
I adhere to the existentialist point of view and, though I was bred a Christian, I've been an agnostic for quite a while now. (ever since I reached the age of reason
I think that the very concept of "purpose" is a human invention - it is artificial and non-existent outside human societies. It's easy to tell: can you picture a walrus contemplating the purpose of life? The only reason we do and other animals do not, is simply that we're smarter. I believe there is no qualitative difference between humans and animals - the difference is quantitative, meaning there's nothing special about us: our intellects are just more evolved.
Another argument in this sense is offered by past experience. Namely, the evolution of humanity from a place of disdainful arrogance (believing the Earth to be the center of the Universe and humans to be the ultimate purpose of Creation itself) to one of enlightened modesty. We now have a vague sense of proportion and we're just coming to realize how utterly insignificant we are in relation to the breadth and complexity of the Universe.
Knowing what we know today, I think one has to be rather conceited to still believe that there is some great purpose for each individual human life, when even the importance of the human species, of Earth itself, of our galaxy even, is questionable given the immensity of Space.
So I agree with Stephen. We decide our own "purpose" - not to be mistaken for "destiny".
We should seek that which makes us feel happy and fulfilled and avoid that which makes us feel sad and empty. To me, that's the bottom line of our species' plain, meandering existence