It's very simple, really. The default position for any claim of truth is not to believe it without evidence. Evidence does not have to be absolute proof, but it does need to be independently observable and verifiable. Now, the bigger the claim, the bigger the requirement of evidence in order to justify belief. If you say you bought a dog, I wouldn't really require any significant evidence in order to believe you. That claim doesn't make any difference in my life, or pose any challenge to my understanding of reality. People buy dogs all the time. I have experiential evidence to back that up. On the other hand, if you said you bought a dragon, I'm going to need you to provide some really hard evidence before I'll believe you.
The claim that a god exists is a big one. It has all sorts of metaphysical connotations, and depending on what specific claims you make about that god, it could have many different impacts on how I view life and its meaning. So, understanding all that, I'm not going to believe in any claims of the existence of god until such time as the claimants cam provide some really startlingly amazing evidence to back them up. To date, all of the evidence and arguments I've seen and heard amount to hearsay, circular reasoning, appeals to authority, appeals to popularity, special pleading, and many other logical fallacies. I have yet to see anyone bring forth one shred of really credible evidence for the existence of a god.