I grew up in the 50s when every man and his dog believed in god.
At school, we were made to pray to mythical god each morning in assembly and every night in class before we went home.
One day, I guess that I was about 8-ish, I announced to some friends that I didn't believe in god and that the concept was complete crap. Friends told their friends and someone then told the class teacher. The teacher, one morning asked the class to raise their hands if they believed in god. I kept mine down. To cut a long story short, I was caned across the hand. My god, that hurt. It must have done because I remember it as if it were yesterday. I'm the most stubborn person I know and from that day on, I refused to believe. Back then, I just kind of knew that god didn't exist without understanding why. Many moons and many books later, I now know why I don't believe.
When and why did I originally become an Atheist? I'm not sure. My best guess is that I never really believed in god in the first place. Back when I was growing up, it was just expected by society that people believed in god and, at the time, it was unlikely that anyone didn't and unthinkable that anyone would ever admit that they didn't. Well, society and I weren't the best of friends and I certainly had no intention of believing or doing something because society said so.
Much later in life, not only did I realise why I didn't believe in god, I also found out that I have Asperger Syndrome, which explains a lot, especially my behaviour. I used to say that I 'suffer' from Asperger Syndrome, but, you know, I now actually enjoy it.
Because of Asperger Syndrome, I see the world as a binary set of processes. Things either 'are' or they 'are not'. Mine is a binary world. Things are either black or white. Greys do not exist in my world. Greys only serve to confuse. Greys are invented by those who intend to fool. If god cannot be proven to exist, and the burden of proof most certainly rests with the perpetrators of the god nonsense, then he does not exist and there is the end of the matter until such time as someone can provide the said proof.