I have heard that some people do not cross over because they feel they have something left to do. Maybe ghost is not the best word, maybe spirit/angel sounds better. I know the spirits/angels that have been or are in my children's, grandchildren's, and my life are looking out for us.
In a dream my DD had about an acquaintance that was a member of her church, he came to her in a dream and told her to tell his family that God forgives. He had committed suicide.
I know there is evil in this world, look at the terrorists. But I know also the spirits we have talked about are good and are here to look after us.
According to the Bible, spirits and angels do visit us.