Why Atheism? You want the simple answer, or the details?
The simple answer is, of all the religions I have seen thus far, mine is “none of the above”. I am not Catholic, nor Orthodox, nor Lutheran, nor Baptist, Calvanist, Methodist, Quaker, Episcopalian, Pentecostal, nor Unitarian. I am not Sunni, Shia, or Sufi. I am not Orthodox Jew, nor Reform, nor Conservative nor Reconstructionist. I am not a member or believer of any of the thousands of religions past or present. Not being a member of any, I am an atheist.
Do I actively deny god? Um… which one? It’s not my job to actively deny a god; it’s the believers’ job to promote him/her/it/them.
What do I believe in? Well, quite a bit, actually–just none of it has anything to do with religion.
The more complex answer?
I remember being a non-believer as a child. I doubt that I would remember not believing something, except that my parents were actively looking for a church, and brought us kids to each prospect (or, in the case of the Mormons, they came to us). None of the churches became “ours”, and we eventually moved from that town without ever regularly attending any regular religious services. At our new home, we joined a church my parents chose mostly for political reasons (local politics, that is; my father had to maintain a certain standard). At first, I resisted going to this church; I would do my best to hide on Sunday mornings.
Then, one Sunday, I had a religious experience. I became, willingly and eagerly, a born-again christian. This lasted for years. I did not see it as conflicting with science or with critical thinking; I firmly believed that anyone who followed the evidence would accept Christianity, just as anyone following the evidence would accept, say, evolution.
Eventually, I found better explanations for every “Goddidit”. Including the overwhelming feelings I had had in my “religious experience.” I no longer believed in that particular god, and had no need to search for another to take its place. I am not bitter, I am not an atheist out of hatred for god, I am not missing some important something in my heart. I simply do not, and cannot, believe, in part because I know more about religion (my former religion and many, many more than I was aware of when I was religious).