Well, I was raised in a very fundamentalist-type of Christian sect. Up until I was about 18-19, I just accepted what I was told and believed it was true. But then my eyes started opening, particularly regarding the terrible things that happen to humankind. Children starve, are abused and murdered, killed in wars, etc. I could not reconcile that reality with what I had been taught about God, namely that he was (1) all-powerful, (2) all-knowing, and (3) loving.
I cannot see a loving parent allowing their child to be hurt or killed if they both knew something was about to happen to them, and had the power to stop it. I have been told that the Biblical view of human suffering is because Satan challenged God and said that humans with free will would sin. God then effectively said, "I think you're wrong, but I'll give you a chance to prove it."
Ultimately, that casts humanity into the role of pawns in a universal game of Truth or Dare. If that is the case, I refuse to worship a being who could be so callous.
That's where my faith broke/shattered. Once that happened, I started looking objectively at the arguments for God's existence. (Or the devil's, for that matter.) I believe the arguments are weak, and I can see far more logic in a lack of a god all together.
Since you asked.