I'm not saying this to anyone on the board (because I have no fucking clue if this applies to you), just some obseravations. I think these are three, of probably a lot, of common reasons people don't believe in God:
Fear of offending people who don't believe what you do. This applies especially to theists who believe in a specific God.
Fear of being confronted by God, because, down deep, we know that this would probably demand something severe from us.
Being angry at God for how fucked up the world is.
I think being polite is incredibly overrated in situations like these. A small example would be a song like "Shaking Hell" which probably offends the hell out of 99% of the population. A larger example would be that the parents of white children in the Deep South in the 1950s were offended by the idea that their children should have to go to school with black kids. But they needed to be offended. An even better example would be that I have no problem offending Pat Robertson or Osama bin Ladin by telling them that their god is false, that I
know for certain, that God does not hate gays or Americans (though he might hate capitalism).
I can understand being afraid of being confronted by God though. When I was 18 I was stricken with scrupulosity (
http://en.wikipedia.org/wiki/Scrupulosity). I couldn't stop thinking about how if God were truly good, he must hate so much of what I do. It was a fear of being confronted that, due to my brain chemistry, got way out of hand. I got on meds, wrote and prayed and read and listened to music like crazy, and was finally in a place where, though still afraid, I could say, "here ya go, God, do with it what you will"...and found out that God is love. and I don't mean God is loving. I mean God
is love. He authored it. He is it. I realized how much comfort there was in being flawed and loved. This is why I say it would break my heart with joy if I saw Hitler in heaven. He, more than most people, probably would have issues with being able to accept he was forgiven.
Being angry with God about the world being fucked up only bothers me sometimes, when I forget that free will essentially dictates that the world will have problems, unless people were perfect, which they choose not to be (ha!).
I don't know if this makes any sense or even has to do with anything. Just thought I'd say it. I don't think these things would immediately occur to the atheist or agnostic as being the reason for their belief/lack of belief. I think it would be deeply buried probably. And I also think there are people who plum don't believe (and I think they will someday, whether when they die or before they die. And I think God's love will be such that it will be irresistible).