Wonderful essay by Ricky Gervais about the difficulty in answering the question of why one is an atheist and what people are really asking with that question.
Why don’t I believe in God? No, no no, why do YOU believe in God? Surely the burden of proof is on the believer. You started all this. If I came up to you and said, “Why don’t you believe I can fly?” You’d say, “Why would I?” I’d reply, “Because it’s a matter of faith.” If I then said, “Prove I can’t fly. Prove I can’t fly see, see, you can’t prove it can you?” You’d probably either walk away, call security or throw me out of the window and shout, ‘’F—ing fly then you lunatic.”
(Via Daring Fireball)