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). 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.
The idea that atheism is some sort of rebellion is a common one, but also quite absurd. I often hear people saying that I'm rebelling because I don't like the idea of "something bigger than me", or I don't want to be accountable to a higher power, and so on. If that was the case, then I would not be an atheist, because I would still believe in the God I was trying to get away from! I do not merely say I am an atheist in order to make life easier for myself - I actually am an atheist. I do not believe in the gods of the Jews, Muslims, Hindus, Christians and all the rest. If I was just rebelling, then surely I would know that God would still be there, waiting for me when I die. The accusation of rebellion seems to come from theists who are simply unable to accept the fact that there are hundreds of millions of people in the world who just do not believe in their god, or anyone else's. The idea that "everyone believes in something" is so deeply ingrained in their psyche, and their own belief is so deeply held and important to them, that the only way they can deal with non-belief is to deny that it actually exists (or they start saying that atheism is itself a religion). It's funny, but I've never heard of a Christian saying to a Hindu "Ah, but you really do believe in Jesus Christ, you're just in denial. When are you going to stop pretending? All this worship of Vishnu is just rebelling against what you know in your heart is true!"
People do not choose atheism - they realise that they have lost their faith and have therefore become atheist (some even fight against it, and it can be traumatic to lose your faith if that faith has been a major part of your life). One of the common reasons for this is that people simply realise that (for themselves, at least) better, more complete and coherent explanations about Life, The Universe And Everything can be found outside of religion - the universe just makes a lot more sense without any sort of God involved. We see no evidence of a Creator in cosmology or biology (which covers just about everything); nothing is known to exist or happen for which the only possible explanation is Divine Intervention. Many, if not all, of the answers given by religions range from vague and ambiguous, through incoherent and contradictory (unless you just have "faith" that they are correct), down to ad-hoc explanations which cannot be tested (or are easily refuted and demonstrably false pseudo-science). Many of the rules and regulations laid down by religion tend to be arbitrary or irrational, and those that are not do not appear to be Divinely Revealed anyway. People lose their faith when their religion has nothing substantial to offer, and better answers, philosophies and ways of life can be found elsewhere.
It should be remembered that everyone, you included, 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? =)
I became an athiest when the world (and indeed the universe) in which I lived was entirely consistent with one that has no God, no Creator, no Guiding Intelligence. There was no valid reason to believe that any sort of a God existed. God became just another supernatural critter that people seriously believed in despite the lack of evidence, like Bigfoot, alien-abductor and lake monsters. I don't necessarily think that all religious people are deluded, irrational nuts (although a minority certainly are - see my feedback pages). I just think they're mistaken, and may very well hold perfectly rational and coherent reasons for their beliefs.
I saw that many people had different reasons for believing in Gods, including (somewhat over-simplified):
* They had simply been indoctrinated at an early age, and the beliefs became so much a part of their life that the idea of questioning or doubting simply never occurred to them. They believe it because they have just always known it to be true. (As mentioned above, people usually end up with whatever the religion of their family happens to be.)
* They feel that we must be here for a purpose. The universe and this beautiful Earth cannot all be a pointless accident, and God is the best explanation. We are all here for a reason, but only God knows exactly what that reason is and we'll just have to trust him.
* A sense of justice. It's not fair for evil people to commit all sorts of terrible acts and simply escape punishment by dying. The idea of heaven/hell ensures that good people are rewarded and bad people are suitable punished.
* A deep objection to being merely "animals". Many creationists, for instance, are offended at the idea that humans simply evolved along with all other animals. God made us separate and special, and we have a "soul" but the animals do not (biological snobbery?).
* Comfort. Many people find it very comforting to believe that a loving God is always watching over them and caring for them, and when anything good or bad happens to them, this is all part of his Plan for their life. Also, religion is obviously very comforting when faced with the death of a loved one. It's much easier and more reassuring to believe that Grandma is free from pain now, up in Heaven with Jesus where she's waiting for us to join her for eternity, than to think that she has simply ceased to exist and the universe will have forgotten her in a couple of generations.
* Afterlife. It seems almost incomprehensible to believe that when your body dies, your existence ends. Sensation and awareness are so much a part of our existence that it is extremely difficult to imagine no longer being able to experience anything. Try to imagine that, for example, a big meteor crashes into your house and instantly kills you at the end of this paragraph. We just cannot imagine not existing or abruptly ceasing to be aware and conscious, and so it is quite natural to think that something will continue to exist after the death of our brain. It's a short step from there to accepting the idea of an immortal soul, and heaven or reincarnation or some other form of afterlife.