I guess I never really believed in anything like like a god. Growing up my dad was an agnostic hippie, and many of my moral values derive from that. Being agnostic he told me and my younger sister what he believed, but at the same time he was always careful to this was his personal belief, and not any objective truth.
My older half-sisters mom, who would often babysit us during our childhood, on the other hand, was a full blown reborn christian nutcase. Because of her, I was always a bit sensitive to religious dogma.
But this only explains why I don’t believe in god, but not why I am an atheist. That final step came a couple of years after starting university, while living in a dorm. One night me and a couple of my dormmates were discussing belief, and they asked me how come I didn’t believe in anything? The were polite about it (in Denmark it isn’t really controversial in any way to be an atheist), and they were not in any way strongly religious any of them, so it was not a case of me having to defend my views.
But anyway, I had to think about it, but arrived at a long argument (which I wish I had written down, as it was a really good argument), which in short amounts to this: My basic values, and the foundations of my world view were incompatible, incommensurable even, with the concept of a higher power. That was it, I cannot believe i god, a god, any god, because that would require that I give up the most important parts of whatever else I believe in.