For me it's simple matter of one's own observations and how he/she comes to a conclusion. The real question (in my opinion) is "Does it make sense for a god to exist?" Whether or not something makes sense to one person compared to another is completely subjective, and therefore one's answer can only be considered opinion. Some people answer by saying yes there is a god/higher power, some people are unsure, and some people say there isn't one (all at varying levels and for different reasons).
I don't believe in god because I don't see why there would/should be one. It seems like an overly-complicated and very human solution to a question nobody can answer. I did not grow up in a religious household, although I did sort of grow up with the idea of a higher power. I rejected the idea fully as I got older. Although I am very firm on my position on whether or not there is a god or higher power, I am willing to admit that there is no "proof" for either position. There is no certain answer, and I am fine with a permanent question mark.
Because there are so many ways of looking at and defining god, presenting supposed proof is unimportant. There are highly acclaimed scientists out there who deal with the origin of everything in the universe who are religious and believe that at the end of the day, everything must've been divinely designed. It doesn't have to do with science vs. non-science. It has to do with one's own opinion. Again, I say "opinion" because it is a subjective question.
On another note, I just watched an episode of "Through The Wormhole" on the science channel where the subject was whether or not there is a god. It's was rather interesting, and I suggest checking it out if you can, if you find discovery channel-type programs interesting.
As a last thought that is somewhat unrelated:
Wouldn't God need a definite, singular, universal, definition to even exist?