Perhaps people believe that ghost has a scary looking face.

I don't scream! I go to shake its hand XD

If there's something strange in the neighbourhood. Who you gonna call? Ghostbusters!!;)

If you have a strong belief in ghosts and then you actually see one, you may scream or run away because you have finally seen that your belief has become reality. You might also be afraid of being attacked by the ghost.

I think this is a good question. Suppose, a little baby see a ghost, what will she do? We've been told that ghost is evil, and will kill us. So we are afraid of ghost. Maybe there's some cute ghost. hehe, who knows....