I think the prevailing theory about ghosts is that they are souls that linger on when they are separated from the body in a sudden or violent way and don't realize they've passed or that they have unfinished business that keeps them from moving on. If that were the case I would think the entire country would be populated by ghosts that we wouldn't even know about. Think of the Civil War for example, and how many Americans lost their lives in a sudden and violent way. How many of them had lives left to live and had unfinished business that would cause them to linger on once the body can no longer support them? How about the Native Americans that were here even before and how many battles were fought amongst warring tribes? What about the Spanish flu or other illnesses that have blown through and claimed many? Not to mention how many people have died who didn't die violently, but who maybe weren't ready to go? I would think that would make for a lot of ghosts and we would be inhabiting space with them all the time without even knowing it.
I have a neighbor who shot himself last October and died in his bed. About a month ago a family moved in there with a young child. They are renters and I don't know if the owner of the house even told them about the suicide or not. Maybe they know and don't care or maybe they don't know at all? I don't know, but they are happily living in that house now and have all moved in and set it up as their home. I walked past their house today (they live next door to me and I have to pass their house to get to mine) and I saw that she had a new decoration up in the kitchen. It was a big L-O-V-E in the kitchen hanging over the stove. I don't think she's worrying about ghosts.
I personally don't believe in them, but I know a lot of people here in Florida, being that this is a tourism state, love to capitalize on ghost stories and have haunted ghost tours and such. It's gimmicky but apparently it works. Nicolas Cage, for that very reason, bought the La Laurie mansion in New Orleans that was supposed to be haunted.