One of the most common questions that theists ask nontheists is "Why don't you believe in my god?". The answer is that we simply do not see any valid reason for holding such a belief. In order to rationally believe any claim, one must first have credible evidence. There is, however, no credible evidence for the existence of a god. So nontheists don't believe in gods for the same reason that theists don't believe in Santa Claus, the tooth fairy, or the gods of other religions.

I contend that we are all atheists. I just believe in one fewer god than monotheists do. :)

When theists understand why they dismiss all the other possible gods and goddesses, then and only then will they understand why I freely choose not to believe in their god(s)/goddess(es).

A misconception held by many theists is that nonbelievers reject religion because they do not understand it. Typically, nonbelievers understand the dominate religion of their culture quite well, often better than many of the believers. For example, here in the USA, where most religionists are Christians, many nontheists are former Christians who have examined the Christian religion carefully and rejected it as mythical. It is precisely because the former Christian was open-minded enough to question and honestly seek answers that he or she ended up rejecting Christianity.