Remember that human ghosts are the spiritual remains of what were once human beings. Like ordinary living people, human ghosts may be nice or nasty. Death does not change the overall personality of anyone.
A human ghost might remain on a property for many years without any indication of its presence, and then become problematic only when a psychically sensitive agent, especially a child or an adolescent, enters the scene. It is possible that a family with young children or an especially psychic adult could move into a property and find that it is haunted even though the previous tenants never experienced any signs of a ghost.
A ghost that has silently haunted a property for generations may suddenly begin to manifest its presence when a living agent who is unconsciously psychic and has a lot in common with the ghost moves in. As an example, a modern family might experience a Victorian lady hanging around the nursery hovering over a newborn baby. This could be the soul of a woman who died in childbirth and mourns the loss of her chance to be a mother. She has been hanging around the room where she died ever since. She has been fixated on this emotion for over a century but has never materialized until another mother and baby came onto the scene. She is energized by the harmony between her own spiritual frequency and that of the mother who unknowingly lends her the energy to manifest.