Reason to believe in God (#1): When I was a boy, I had many powerful spiritual experiences that convinced me there was a God. As my education made more scientifically-minded, I started to think that the source of these feelings was psychological: like with good fiction, imaginary ideas and images can produce very strong feelings.
There are a couple problems with this line of thinking. First, images in, say, a good movie may be imaginary, but they are so powerful because they reflect something real--heroism, sacrifice, redemption, love, etc. I often feel something very special when I am outdoors or stare at the night sky, and nature is of course real. Santa Claus or the Bogey Man may inspire some feeling, but what emotions they do inspire come from real things that the characters bring to mind: abundant gifts under the tree or child predator. You might say that God is imaginary, but He inspires strong feelings because he reminds us of our fathers or other real heroic figures. This does not square with my experience at all: my feelings for my dad or for George Washington are simply in a very different, inferior class.
This brings me to my second point that the class of emotion I have felt when contemplating God simply cannot be touched by any other kind of feeling I have had. Relationships, struggles, imagination, and nature generate a rich supply of feelings, but experiencing God is categorically different. It seems unlikely to me that a lifetime of experiences this profound would come from something purely made up.