I had a discussion with an atheist once, and he concluded that the only purpose for life is to pass on our genes and try to make the world a better place.
I told him you have no hope but to be a pile of compost.
He finished by saying goodbye "from one pile of compost to another."
He was a nice guy, not like some who are snarly and condescending.
Al Serrato over at please convince me has an article concerning purpose from the atheist's viewpoint.
He starts the article:
“There are asedno atheists in foxholes.” Or so the saying goes. Today, that probably has more to do with the scarcity of foxholes than it does with the scarcity of atheists. Indeed, the growing ranks of atheists include some who would like the military to allow them to designate “humanist” on their dog tags and official records. Just as a Catholic would wish to be specifically labeled and not bunched with other “Christians,” they argue, so too the “humanist” wants his “positive philosophy” to properly reflect what he believes."
Later he adds:
"Atheism posits that we are accidents of evolution, with no transcendent or lasting purpose. The universe just happens to exist and we just happen to be the unintended byproduct of a string of events which were set in motion randomly untold billions of years ago. We pass our brief moments in the sun, and in the end, we simply return to dust. The quality of the lives we lived, and our desire to continue thinking and growing and… being…count for nothing. There is no ultimate arbiter of right and wrong, neither punishment for evil deeds nor rewards for the good that was done."
Truly life's purpose is pretty bleak by their view.
You can read the whole article here.