It's a TERRIFIC question, and the only answers we can offer are theories, since that's the best we can do in this field of study.
Here are a few reasons spirits may "hang out":
- They have a message to get across or business to finish. Maybe a spirit cannot rest because there is a misconception about themselves that they want clarified. Perhaps it is believed they killed themselves, but they didn't, and they only want the truth to be known. Maybe they want to make sure that a certain part of their estate is handled the way they want it handled... it might stay around to make sure it goes as planned.
- There is a friend or family member who still needs them. This is one of the sadder situations. There might be a survivor who is so grief-stricken and cannot bear to continue, so the spirit feels the responsibility to stay with that person until they are strong enough to move on. Only then, can the spirit move on. This is one great reason to go ahead and grieve a loss of someone you love, but be clear that you don't expect or want them to stay and "babysit" you.
- They may not know they are dead.
- They may want to just witness something... for example: a mother who dies in childbirth may want to stay around to see that child grow to adulthood.
- They may just be frightened or confused about moving on.
- They might have such strong positive memories about the times they spent in a place while they were alive, that they want to spend more time there after death. It may feel like a safe haven... like home.
- And, although it isn't as common as all the reasons above, a negative entity can sometimes keep spirits in a place against their will. Then you have a "bad guy" trapping spirits who are trying to leave.
Just to clarify, just because a spirit is around doesn't mean it is bad and "unworthy" of moving on to a better place. There are many reasons they may hang around, and only some of them are listed above.
A terrific resource for more information on how to identify what you are dealing with, and how to proceed is Michelle Belanger's The Ghost Hunter's Survival Guide, a book that I highly recommend. This book has oodles of info for the aspiring ghost hunter.
Important to note: Just as we should not force our opinions on others in the living world, the same goes for spiritual entities, except in extreme circumstances. There may be some reason an entity is still around, and if so, it would not be right of us to try to force them "into the light." IF it were a negative entity, that's a different story, but that's a whole different subject.