My mother brought my siblings and me to a Presbyterian church off and on for a couple of years when I was very young. Other than that I was never raised to be religious. Throughout my childhood and teen years I believed in God but it was a rather vague belief and it didn't do anything to shape my life. I believed in God only in the sense that I would give a positive response if asked. I didn't subscribe to any religion because I thought (and still think) organized religion is subversive and caused a lot of evil. I wanted to believe in God but on my own terms. I really closed my eyes to the idea that God may not exist. It was a silly thought to me. I told myself "Since everyone believes in some sort of god there must be one and they're all praying to it in their own different traditions and through their own understanding". I latched on to this idea for a long time (but now know the logic is fallacious and is an appeal to popular belief).
After high school I joined the army and in 1998 I was deployed to Bosnia. There I saw enormous cemeteries, too many to count and in some cases too large to stand and view their borders, filled with the victims of the war. I saw the remains of what once were houses but since were marked with a spray painted x to signify that they were inhabited by Muslims and then riddled with bullets by Serbs. Other houses in the same neighborhoods were leveled to their foundations from rocket or mortar attacks. I saw old gray women in the streets forced to beg for money because they lost their legs to landmines hastily left in their backyards during the war. I saw other women standing in market places holding up pictures of their sons or husbands, men who died in the war, asking us soldiers if we knew where they could go for financial aid. A part of my job was to give mine awareness classes to elementary school to high school aged children in their classrooms. We had deactivated landmines to show the children so they know what to avoid. In one class, filled with twelve or thirteen year old children, one of my teammates held up a small antipersonnel mine and asked if any of them had ever seen one. One child held up his hand which was missing three fingers. He had to give no description of his account as his proof was his deformity. On multiple occasions my team and I were approached by Muslim extremists, members of the Mujahideen, who yelled at and spit in our faces and wished for our deaths, none deterred by the loaded M-16 rifles on our shoulders. It was in Bosnia that I asked myself if I was absolutely sure I believed in God.
I didn't think there was any way to prove or disprove God's existence so the question lingered but I didn't feel compelled to try to answer it. When I got back from Bosnia I started noticing things that didn't accord with my view on God and admitted to myself I was agnostic and remained so for many years. On my first date with my current girlfriend I told her I didn't believe in God although I didn't call myself an atheist. She asked me why I didn't believe in God and I wanted to give her a good answer. I then started a search for answers. I asked myself what still made me believe in God. The argument from design struck me as valid but then I studied evolution and learned natural selection is a mindless process that has no goal in mind and we and every other species are products of it. The cosmological argument seemed valid but then I studied cosmology and found an explanation of how stars form from clouds of hydrogen and create the heavier elements that make up planets and everything on them including every atom in my body! The more I learned the less reasons I had to believe. The only issue that remained is if God doesn't exist there would have to be a reason why the universe does. Then I learned about the Hawking-Hartle Wave Function Theory that states the probability of our universe coming into existence uncaused is 100%. Dr. Stephen Hawking started work on this theory about 15 years ago and it's been verified by observational data. Even if this theory turns out not providing any answers to the origins of the universe I am convinced by the direction of progress we mere humans have made in gaining an understanding of reality one day we will have all the answers.
That is how I became and why I still am an atheist.