Logic.

(I can't find one thing to add to the 15 wise comments below!)

Because I lie my life based more on verifiable facts than by blind beliefs.

Because religion makes no rational sense.

I thought about it, and the god idea just didn't make sense.

Read a lot of books, looked at other mythologies that were popular before biblical times, and realized, oh, it's a way to gather people together in a church and tell them things to blindly believe, and warn them not to question it, or they'll burn in hell...

That way the government can control the citizens better. It's not for me. I prefer honesty, reality, and natural causes and consequences.

I need no reason for "I don't believe". I would need a reason to believe a claim, not a reason not to. non-acceptance is the default state for any claim, acceptance requires a reason.

Because I've always been

I am agnostic because there is no proof that a god exists. When there is proof, I will gladly change my mind, but until then, I'm going to base my beliefs on science and personal experience.

Oh, for heaven's sakes! I don't have to justify my atheism to anyone.

This is my reply to Christians who hassle me. Choose one. All are true:

I am grounded in science and reality.

At age 13, I became an atheist when I realized the Bible is just a book of stories written by men.

I don't believe in an invisible being that resides somewhere beyond the clouds.

I stopped having imaginary friends at age four.

well my parents were against child abuse so .....

Ever go shopping for something (a book, a car, a cd, or anything else) only to go home without? Guess it was something like that for me. I'd look around and see people going to church when I was young and wonder what it was that drew them, so I'd look into the Baptists, and the Mormons, and the Catholics, and the Jehovahs, only to be confused and unable to discern how one could possible claim any advantage over another. So, I zoomed out, examining faith, and morality, and the notion of good vs evil and came away from that empty, seeing no connections, or substance upon which any claims for religion or superiority can be made. Growing up, my family was noncommittal toward organized religion, but eventually found some branch to latch onto. I didn't. To my knowledge, they stumbled upon something that kept their attention, whereas my curiosity and readings kept me from doing the same. I was never really adrift, I just didn't really have the affirming experience as early as some. I went shopping for a religion and found it wasn't something I wanted or needed.

I was all in on Santa Claus and Jesus as a kid. But in my child mind Santa clause was more important because he brought me real gifts every year. So he was the boss. When I found out that Santa was BS then it took less than a second to doubt all of Religion. Now I'm certain all 4200 plus religions on earth are false. I'm agnostic because science has not adequately explained the origin of the universe or the multi verse yet.

Because the god thing never made sense to me, and the more I know about the universe the less the god delusion fits in.