I stopped believing because I simply saw no evidence that he or she exists. If I had not been raised with a belief in a creator and been told that he existed as an adult I would have not been convinced by the evidence at all. Even if the God of the bible did exist, he hasn't communicated with mankind in thousands of years, and the book he supposedly had written about himself is a contradictory mess that describes a jealous, petty tyrant, so why would I worship him if he did exist?
The evidence we evolved is overwhelming, our history is written in our DNA, so that explains how humans came to be, and that is a lot more believable that the genesis account, which couldn't have happened the way the bible said it did. A literal, earth wide flood? There are so many logical inconsistencies to that it doesn't merit discussion. We don't know (yet) how life started (abiogenesis), but I have full confidence it will be discovered and explained by science, whereas no one can explain how God came to be.
I used to wish I could believe in God, but I can no more believe in him than I can believe in the tooth fairy, as both seem equally unlikely to me. I have made my peace with that, feel happy and content in my life and optimistic about the future.