Whenever I'm asked this, I think about how to respond. I've written blog posts about it. When asked this recently, I said what I had most recently written in a blog post (something about not wanting to be judged). But then, that didn't seem like a reason not to believe, but rather a reason not to want to believe. I had forgotten what I wrote in that post about free-will, which makes more sense as a reason, but as I was thinking about it, I thought of another reason.. well, this reason is along the lines of the story I tell in this post (something about comparing the feeling of the spirit to the feeling of understanding something in physics). The core point of that story can be summarized as follows:
I decided that I shouldn't be asking "why not be Mormon?", but rather be asking "why should I be Mormon?". And the trouble is, the only reason I can find to be Mormon -- and even most Mormons will tell you this -- is by feeling the holy spirit. But I don't know how to know whether I'm feeling the holy spirit, or just feeling something coming from my own brain. I think that the brain can produce feelings which one could interpret as coming from a divine source. So.. I need something else to convince me that Mormonism is correct, and nothing else has.