People become atheistic for many reasons. A popular misconception among theists is that we have some terrible experience (lose a loved one, for example) and ask "How can I believe in a God that allows this to happen?", or start to hate God for allowing it to happen. It should be noted that in order to hate God, you actually have to believe in Him. People who claim to hate God (for whatever reason) are not, by definition, atheists. An atheist may hate a religion, or hate what people do in the name of a God, but an atheist cannot hate something that she does not believe in (it would be like hating dragons for eating princesses).
Another misconception is that people choose to become atheists. It's as though we say to ourselves, "Well, I really do believe in God, but I'm just going to pretend I don't believe.". Atheism is not a choice - you don't wake up one morning, flip a coin, and declare yourself to be atheist. It is not a teenage fad, nor is it a rebellious act (although some people are driven to atheism by constant preaching - see below). It is not an arbitrary decision - "What shall I have for breakfast today? Erm... toast. What colour carpet should we choose this year? Erm... green. What god should I believe in? Erm... none." - it doesn't work like that.
It should be remembered that everyone, you included, gentle reader, is born an atheist - babies do not believe in God. Your religion often depends on your upbringing - if your parents are Southern Baptists, you will probably also be a Southern Baptist. If your parents are Muslims, then you will probably be one also. Which God do you believe in? I'm betting it's entirely dependent on geography. It's a bit of a lottery, isn't it?
Aren't you lucky to have been in a country that just happens to worship the One True God?