No reason justifies disbelief.

[possibletarian]

I didn't say there are no reasons to believe, after all simply saying 'that you want to believe' is a reason. In the same way saying 'I don't want to believe' is also a reason, but both are simply declarations of what you believe to be true.

What I actually asked for is proof, not reasons

Thank you, I am interested in people's reasons. 

I'm sure you know the drill about the word "proof." It's only for math, pure logic, etc. Though I'm perfectly happy to use the word in reference to other areas if we use it to mean something like "evidence which I hold to be overwhelmingly persuasive." 

This, again, means that we have commitments or judgments about what constitutes good evidence, what constitutes proof, etc. These are the underlying reasons people reach the conclusion that there is no reason--and no proof--to believe. Sometimes people take the standards of judgment to be self-evident and conclusive, but they are still concepts which we use to arrive at a considered atheism.

Interesting, what do you mean by 'considered' absence of belief in a god ?
And if you are a believer yourself, what standards of evidence and proof did you find convincing and why ?