Do you believe in ghosts? Why or why not?

Yes, but in full honesty, the term "ghost" is in and of itself misleading.

There are poltergeist which are the psychological manifestation of stress. Typically where somebody is so stressed out, it affects the environment to make it seem like fire, lightning or things are moving about in an odd fashion. Whereas it is likely the stress of the situation itself is mostly what is causing most of these occurances.

There are emotional imprints, where an emotional event happened with so much force, that to all those that are there, they seem to think they can see it. Usually some battle or what not--or some extreme event.

Of course moving into more real ghosts, you have ethereal husks. The Ethereal body is what allows the Astral self to attach to the Physical self. The Astral self is able to make multiple Ethereal bodies and move them independantly of one another. Typically these are shed when the person dies, after the body itself. Usually the job of the psychopomp for this particular one. The Ethereal body eventual gets eaten by nearby creatures.

Then we get in Demons. The term Demons simply refers to any spirit that does not have the rights to be in this realm. Not just fallen angels--but any spirit on this plane, that does not have proper clearance.

Good enough answer?

As per why I know this information--mostly to properly confirm paranormal cases with the utmost skeptical ability. As most cases are fiction--but with this cultural knowledge, it makes it a bit easier to confirm what is going on--or most likely, not going on.

I do not want people constantly asking me about Grim Fandango