Atheists, why do you not believe in a god?

I am a Christian and have always wondered why some people don’t believe in God, or any god. A kid at my school is an atheist and says he believes in ‘science’. Does he really think that SCIENCE made everything that did, does, and will exist? Atheists, why, really, why do you not believe in a god? What do you believe in? And don’t just say “science” or “logic” and please don’t give me a lecture on how my faith is wrong or ridiculous or something in your opinion. Please, give me a reason why you believe/don’t believe in what you believe or disbelieve in. And, please don’t ask or talk about my beliefs because I wrote this question to hear your answers, not to be back talked to.