Personally, the original reason I believed in God is that an adult told me he existed when I was little and, being little, I took their word. But over time, I've listened to people talk about their experiences with God and seen it with others. I think I've seen Him get me through a lot of stuff the past few years that I don't think I'd have been able to make it through alone. You could say that I got through them because I worked hard, or just because believing in a higher power has some effect psychologically, or that there were coincidences involved, but when I put it all together, those reasons just don't work for me. But if I had to give just one reason that I believe God is real, it would be that He told me so. Audibly.
what makes you believe that God is real and not, say, Zeus or Allah or whomever?