I've been an atheist all my life. Both my parents were, but apostasy, especially on my father's part has shaped my upbringing, my life and my sense of family.
Intellectually, I have always found gods and the general stuff of superstition patently absurd, but I was not a militant atheist until September 11th, 2001.
Why are you here?
Well, Mummy met daddy and they fell in love..... Oh! Here specifically! :-) Atheists need to support each other and make their lack of belief in gods public in order to arouse the curiosity of believers and hopefully educate them (short of proselytising), and to offer moral support to those who feel they may be in difficult positions with family/community because of their apostasy.