Quoted

A wandering ghost is a dangerous kind of entity, because he or she might decide to wander back … herein, of course, lies the very root of our horror of ghosts. They are feared not because of what they are, but because of what they remind us of being. We are frightened of ghosts because we are frightened of dying. That is the first thing to be said about the matter: that ghosts, like corpses, are unwelcome because they are memento mori.

– Roger Grainger The Social Symbolism of Grief and Mourning, 28-9