There's a significant difference between positing "There is no god" (which is not what I say in general) and saying "I do not believe that a god or gods exist" i.e I say, so far I have honestly looked at the whole picture and there is nothing that I see that compels me to believe that a god exists. i.e. I don't know for sure but due to the lack of evidence the probability that there is a god is quite low.
Here's a nice analogy for you: imagine a court of law. You have the accused in the box, the defense attorney, the prosecution and the jury. As you probably know, the jury doesn't need to have proof that the accused is innocent. The burden of proof is on the prosecution's shoulders to prove the guilt of the accused. If the jury is not convinced that the accused is guilty, they cannot declare him guilty. This is vitally important. You're innocent until proven guilty.
Same for the question of the existence of a god or gods. I'm the jury. You're the prosecution. The burden of proof is on you - you have to prove to me that there is a god. You're positing that there is (just like the prosecution is saying 'the man is guilty'). As a honest member of the jury, I have to analyze the evidence and make a rational and honest decision. It would be dishonest of me to say I believe when I don't. It's not a choice. It's just a product of my thinking honestly about the question and coming to a natural conclusion. Until I'm presented with sufficient proof, I will continue to not-believe, just like the jury can only declare the accused 'not-guilty, your honor'. The jury doesn't say 'innocent'. They say 'not guilty'.
Saying 'innocent' would be the equivalent of positing that 'There is no God' and the burden of proof would then be on my shoulders. I will not say that because I don't know for sure. I'm happy to stay with 'I don't know' and get on with my life happily, than believe without good reason.
Now on top of all of that, if you read my blog entry 'Back to Normal' (http://mauritianhumanist.blogspot.com/2012/08/back-to-normal.html) you'll understand that on top of all that I just explained, I am asked to believe in this good, loving god, after all the atrocities that are in this book which is supposedly inspired by this same benevolent god. Have you read the bible from back to front? This god in the old testament commands you and instructs you how to treat slaves, to rape and murder etc. and I'm being nice. Just spend 5 minutes on Google or go there directly (http://www.evilbible.com). It's all in the book, I promise you. Why would I believe in such a monster? No thank you. For me it clicked a long time ago - It looks very man-made to me. Nothing godly there.
This life is probably all that we have and I'm going to make the most of my short time on this planet with peace of mind and as much joy and happiness as possible without the need for a god, heaven, hell, sin, forgiveness and all that jazz!