The short answer is: a severe lack of evidence for god, as in, none.
The long answer follows.
First off, one should know that I come from a very religious background. My grandfather has been an Apostolic Lutheran pastor for my entire life, and my mother became "born-again" at some point after getting pregnant. When I was a child, the family would play "Bible Trivia," and I kicked butt because I enjoyed me some Bible stories.
Around about the seventh grade, I started thinking, and one thing that kept going through my head is that the deck was stacked against humanity from the start, if one believed the book of Genesis. God was omnipotent and omniscient, so he knew exactly what sort of creation he'd made, and the consequences thereof. So, here's Adam and Eve, with one rule to follow: don't eat the fruit of the Tree of Knowledge. What follows is well known of course, the "Fall of Man." What got me though is that God should've, indeed must have, seen it coming from the very start, and took no steps to prevent it. There are many that could've been done: don't let Satan in the garden, don't make the bloody Tree, make sure Adam and Eve really are obedient and not prone to disobeying just cuz of a few honeyed words, just to name a few.
What this would mean then is that God wanted it to happen, knowing full well he was still going to lay the blame at the poor human's feet and punish them accordingly with pain, misery, and no access to immortality (the given reason for being kicked out of Eden is not the sin itself, but so as to keep them from eating of the Tree of Life). That's an eyeopener for a child of that age. That made God out to be someone rather despicable, as opposed to the loving, forgiving deity I had been taught to worship.
Another way in which the deck was stacked, according to the teachings I had received, is the simple math about the causes of sin: the devil, the world, and our own flesh. The only saving grace was to pray to Jesus to make you a good person, and give you strength to resist temptation. "For all of sinned and fall short of the glory of God" Rom3:23. Well, yea, when things are set up like that!
I'm pretty sure, though I don't exactly recall, that before I concluded I was atheist, I concluded first that God just flat out didn't deserve my worship. And this led me to believe that there was no God. Yes, I know, crappy reasoning. God could've existed but really just been that rotten. I make no excuse, except that I hadn't been taught critical thinking in my life.
I won't go into all the details of the teenage years that cemented the "God, if existing, is rotten" idea, and the conversations I had with my Grandfather during Youth Bible Study meetings that didn't help the case for religion. I will say that I gained a better understanding of evolution during that time, which my family's church views as the enemy (when my Mother gave me a subscription to Discover magazine for my 13th birthday, she warned me that if I started talking about evolution being real, she'd take that subscription away -- I kept my mouth shut), and this understanding helped further the belief that God doesn't exist.
I backslid around the time I was 18 or 19, when I had a powerful emotional experience that I believed was my soul touching another. This I took as proof that souls existed, which meant there was an afterlife, and more in heaven and earth than were dreamt of in my philosophy (thank you Shakespeare). I spent the next few years trying to understand that experience, and figure out which of the New Age beliefs, or combination thereof, were right. I still viewed the Christian god as horrible, so I wasn't about to go there! However, I continued to read about science, one of the loves that has remained with me since early childhood, and eventually, I simply couldn't maintain the belief that anything genuinely spiritual/supernatural had actually happened to me, or the belief that there was any sort of deity looking down.
So, I became agnostic. Some more time passed, some more study took place, and eventually, all I could admit to is "atheist." Frankly I find myself more satisfied with the evidential conclusion then I did with my past views. I have even come to view the world with more wonder and awe than I did when I believed in the supernatural. Perhaps in a future post I will describe more fully that wonder, that awe, that delight that I experience when looking at the universe through the lens of knowledge.