The responses so far to this have been really good. I would like to hypthoesize that there probably are some psychological reasons why people believe in God. (Yes, a lot of people are raised to believe their religion, but in the U.S., a lot fewer people "practice" their faith, by raising their kids in Church, than merely "believe".)

I think the psychological reasons are "hope", "fear", and "comfort". People hope that there is an after-life, fear that death may be final, and are comforted that they will not cease to exist after they die. They hope that life is fair, fear that bad people will get away without being punished, and and are comforted that god will take care of it. They are comforted by having an all-powerful father who loves them, and will look after them. They fear bad things happening to them, and hope and are comforted that God will protect them. They fear the unknown, and are comforted that the answer to all unknowns is "God".

Hope, fear, and comfort. There are probably a lot of reasons, but I think that sums it up.