I am an atheist because (a) I don't find any of the specific organized religions that mostly major on the assertion that all the other religions are false even marginally convincing. (b) I don't find the Aquinas-style uncaused cause argument for some kind of creator convincing. (Please note that I am not a "strong" atheist in the sense of one who believes that it has been proved that no God exists)
Of course the fact that I was brought up as an atheist probably contributes. As I've stated in my arguments with ConCrusader I contend that religious belief is not largely a matter of choice.