Nearly everyone has a relative who has died so why do some people see ghosts and others don't?

It depends on how in tune they are to the spirit world. It also depends on whether they believe in life after death. I think that ghost show themselves only after thinking they can trust the home owner to help them. Usually people throw them out with charms and enchantments set by priests. I would never do that because I think it's wrong and that they should be helped so that they can move on.

Many people are tuned into the spirit world one can say. Think of our world like a radio. Most people are tuned in at 100.1 for example. The spirit world is just one channel away-100.2 and therefore very close to our world which explains how they are able to interact with us. People who are born with the gift to speak to those who have passed are able to turn their frequency to the 100.1 channel as well as the 100.2 channel. This ability-known as Mediumship-can be learned through rigorous training, but it is achievable.