If it is not certain that death ends personal identity and memory, then almost nothing that man accepts as true is susceptible as proof. Believe that life is worth living, and your belief will help create the fact. Actually they don't, but this is a good rhetorical device, so I'm sticking with it. After all, Christianity has been good enough for all sorts of people for almost two thousand years. There are many reasons why people become atheists.
Some people become atheists in reaction to a strict religious upbringing.
Occasionally, people like this are in two minds about atheism. Sometimes they return to the church. But no matter how much some people flirt with the idea, they are not atheists if they still believe in God. We define atheism as the lack of any belief in any god. A true atheist can no more believe in God than believe the earth is flat.