Why am I an atheist? To be perfectly honest, my upbringing contributed. Most people stay with the religion of their childhood, and I stayed with the non-religion of mine.
Having said that, I always felt in the minority, and this kept me from uncritically accepting what I was taught. In fact, for a large part of my childhood, I wished my family was Christian so I would be normal like my friends. I had a devoutly Christian friend in primary school, and she convinced me to start praying regularly, and I attended Christian youth activities with her. The youth activities lasted many years: they were fun, although I never really took them seriously. They praying didn't last, maybe a week. At age 10 it didn't take me long to acknowledge that I was talking only to myself.
Fast-forward to when I finished Uni. For the first time in years, I had the time to devote to reading for pleasure, and now I craved knowledge rather than the fiction I had devoured as a child. The first topic I jumped into was religion. I remember thinking as a child that if I studied all the world religions, I might find one that fit me; I had felt incomplete without a religion. Now I read The God Delusion, God is not Great and The End of Faith, alongside The Case for Christ and The Case for Faith and many other books. I tried to read them all with an open mind, but my mind snapped shut when in The Case for Faith, one of the interviewees used the existence of witches as a part of his argument. I read John Shelby Spong, and liked his outlook, but found no reason to accept his beliefs. I read The Demon Haunted World. I watched documentaries (many by Dawkins), and any number of debates on religion. I watched some horrible Intelligent Design videos my in-laws gave me. I progressed onto biology and physics. The more I researched, the more absurd religious beliefs seemed.
It is true that my upbringing biased me towards an atheistic stance, but it certainly didn't determine my adult atheism. The household I grew up in was center-right, and I am now quite firmly in the left. My views about most political topics stand at odds with at least my father, and frequently against both parents. My naive views absorbed from my family have changed in most things, and so too have my views on religion. It's just that with a much deeper and more nuanced understanding, I am now confident in my atheism.
As for my reasons for being an atheist? They're very simple. The evidence for any god is about as compelling as that for the Loch Ness Monster.