It took a lot of time and thought, nothing in particular spurred it on. I considered myself agnostic for a very long time.
I still do on some level, but lean strongly towards atheism now. I'm just too logical of a person. It just doesn't make any sense. At all.
First, the idea that we're in any way, shape or form "special" in this incomprehensibly vast universe is just plain laughable. It's like saying the Sun revolves around the Earth.
Second is science. I know that there are many religious people that believe in science, some perhaps even wholeheartedly. But in my opinion, it takes someone who does not think for themselves to not look at the mountain of evidence that basically shows we are not special.
Third is religion itself. Of all the thousands of religions that have existed in the last 100,000 years.. Have you picked the right one?

If that doesn't make someone realize the absurdity of it all, nothing will.
Religion was created by humans to control other humans, and to try and explain things to the best of their ability at the time. It is not needed in a modern society.
Edit: I would like to add that my Dad, whom I lived with during my childhood, is very religious though not in a church type way. He basically has his own version of Christianity, which I find typical. Anyway, he essentially taught me how not to live.. Just like any good father should.

Asking God for things clearly does not work for him, and I'm sure this had a big impact on my own beliefs later in life.. Though I'm sure I would have arrived at the same conclusion, it probably would have been a much longer, different road.