I came up with this /fringe/ backed theory while laboring at work. I had a second one but it slipped my mind by now. Feel free to post your own theories so long as they have at least a little bit of basis in fringe knowledge.
Ghosts are believed to be an echo from the past, an almost mindless entity that repeats the same general things over and over again. Or ghosts are also thought to be "stuck in the past" and "need closure before they can move on". They are believed to haunt places where they experienced great trauma, or had great love/passion for.
By no means do I claim to be an expert, this is just me trying to piece my knowledge together and make connections.
Ghosts(the mindless "spirits"/echos) are actually just thought forms unintentionally created by the person experiencing extremely strong emotions about something, usually in the process of getting murdered. This is why they "haunt" the place they died in a lot of the time. As we know, great amounts of loosh would be released during a traumatic event or from other strong emotions, even from the opposite polarity(positive). That loosh is focused towards one thing/event/place, thus creating a thoughtform there which continues to linger after it's creator dies. Rarely do ghosts ever materialize into the image of their creator, they are usually formless clouds/smoke/mist/rough shapes, etc. That is because the creator did not imagine a form for it, it is just the product of a surge of raw emotions being poured into a thought.
These "ghosts' can continue to exist for long periods of time because they feed off of the fear of people who see them.