You're coming from the angle that all paranormal events are infact the ghosts of humans who have passed on. There are other entities then just human ghosts.
Anything on this subject is just speculation. There isn't even a concrete definition of ghosts in the paranormal world of study. Some will say it's the dead souls that won't move on, some say it's residual energy. So it all boils down to personal belief & opinion.
I have several different theories. One of my theories is that time is an illusion, that everything (past, present & future) is happening all at once but our brains can only process things 'chronologicially' one event at a time. So some of these 'ghost' sightings may be an event that has 'looped' over the 'present' and is being received by human or electronic perception.
Scientific fact: Energy cannot be created or destroyed. When we die our energy must go somewhere. Does it take on a 'life' of it's own as a disembodied spirit? Maybe. Does it retain it's original human consciousness? Maybe, assuming it had something to live for to remain in the land of the living.
Long story short, I don't believe ghosts are mentally ill. I believe they are here because there is a motivation that keeps them here. Just as most people have a motivation to keep them from killing themselves. Does that make most people OCD? manic depressive? ect.... no, it makes us human. i.e. I believe that ghosts who remain are attempting to hang onto what threads of humanity they can and have no interest in ascending or being reincarnated.
That's my 2cents, don't spend it all in one place.