People believe in God because they feel the need to have some kind of framework to base their life around. I know because I was a Christian and strived for belief up until I became a raving libertarian nutcase, at which point I didn't need religion to give my life direction any more.
Jachie: But it's not mere chance, more process of deduction, as each necessary part of life was created and built upon. For example cells appeared first as just membranes with not much inside, then some developed chlorophyll, mitochondria etc. Then cells began to interact with each other, creating multi-cellular organisms. Essentially this means that the chance because a case of one in 10x (where x is the number of alternatives to a membrane, or a cell as we know it, whatever), rather than x^10 and hence it seems somewhat more plausible than random chance.
EDIT: To rephrase. Looking at humans. You don't have teeth out of random chance, you have teeth because one out of a million prehistoric animals had them, and it was advantageous, so they flourished. Works the same for pretty much every other aspect of your body.