Well obviously I don't speak for all atheists but I simply stopped believing in god as I got older, I believed in him when I was nine... as most nine year olds believe in santa.
I didn't really think about it untill I was 14, I just didn't have any reason to. And then, I'm not sure why, I think I learned something in school that didn't jive with the flood story... So I did a bunch of research, starting with the borgs stuff, and I wasn't satisfied with their super vauge answers. So I looked it up on the INTERNET and realised that it was completely obsurd to believe in the flood, without believing in evolution (Ironicaly the animals would have to evolve WAY faster than scientists think they did in order to make the story work.)
I was shocked, but I liked what I learned. So every week during our bible reading I would look for something that I wanted to check the next time I had the chance, and pretty much every time I checked, the bible was wrong, or at least neaded a heck of a lot of explaining to make it believable.
And then there were the little things that just didn't make sense, like: How did the apostles know what Jesus prayed about when they were all sleeping? What the heck is up with the talking donkey... didn't that warrant an explanation of some kind? Why did god nead to confuse the laungages unless he felt threatened? (Which is just what the bible says...) Why did god ALWAYS punish other people for someones mistakes? Like David multiple times, Akan, Pharaoh, Adams decendants, etc. There's a pile of things that are unexplained and off the wall.
By the time I was 15, I was completely convinced that the bible was balogney... Then I researched evolution and found out that it makes WAY more sense then the society let on. And as it turns out, there a perfectly acceptable scientific explanation for darn near everything!
And since I have never seen any reason to believe in any kind of god at all, I simply don't believe in god. Now that I'm 17 I'm officialy and atheist.
So I'm not an atheist by default, (Even though I was technicaly born that way.) It took a long time and a LOT of learning and research.
I definately reccomend that everyone look at this site... even if for no other reason that to see how atheists think... Because we definately have a different thought pattern than most people: (Well I do anyway.)
http://www.ebonmusings.org/atheism/
I wanted to say a lot more, but I have GOT to go to bed. I'll probably be back tomorrow.
LtCmd.Lore