Reasons
Intellectual
Most atheists would offer some of the arguments on the following pages as their reason for deciding that God doesn't exist
Non-Intellectual
Many people are atheists not because they've reasoned things out like that, but because of the way they were brought up or educated, or because they have simply adopted the beliefs of the culture in which they grew up. It's the same for many believers. So someone raised in Communist China is likely to have no belief in God, because they rarely if ever, meet a believer, and because the education system and pressure from the people they meet make being an atheist the natural thing to do.Other people are atheists because they just feel that atheism is right. In the same way, many people of faith hold their beliefs because they just seem right to them.