'how come some people can see and some can’t' - Probably because people have different sensitivities at any given time.
Just think of the shamanic way. A shaman eats or smokes certain herb and then, he can see the spirit of a dog while other people can't.
Seeing a ghost is a good thing. Now you have a reason to believe in the afterlife. Or in the paranormal. It is not going to bite you.
