Why are you an atheist?
Because I'm not a theist. That might sound flippant, but it's not. I didn't begin my journey toward atheism with that question. It seems to me there is question of far greater importance than that trivial query.
The first question I asked when I thought about God, was how do I know he is real? This is the question at the very top of the importance pyramid. It's important for at least two reasons. First, if he is real, it has important implications for mankind and my personal life. If he is not real, he has no import at all. Second, the problem of existence demands to be questioned by any half way rational person, because God leaves no tracks. If you are going to believe something, knowing (as opposed to just believing) is absolutely critical. Otherwise you are just waving your philosophical arms around in empty space.
To answer such an important question, you must begin sifting through evidence, and objectively discard anything without substance. Unfortunately, for me as a Christian, after years of sifting, I had nothing. I didn't bother looking at evidence against his existence. I only considered evidence for, because that's what is important. I could find nothing of substance. Hence, I couldn't prove to myself God was real, and I couldn't believe in something so insubstantial by my force of will alone.