Ghosts are stupid.

The concept of ghosts (and spirits, for that matter), just like any other superstition, is stupid. Ghosts are believed to be ''souls'' of (usually) people that either are unable to transfer into some sort of another plane of existence (in this case, an implication is made that there is such a plane), or are voluntarily staying on earth, usually near the place they died, whilst not having their physical bodies (and, thus, such ''beings'' could not have any sensations, at all), for some fucking reason.
The day anyone proves me that such thing as a soul exists and is actually of any use at all, I'm going to chainsaw my face off. Soul is a superstitious concept, made to reinforce the ideas of reincarnation, afterlife, ghosts/spirits and so on. Of course, people thought soul to be the driving force behind every action of a human being, but nowadays, we know that it's a bunch of impulses and muscle reactions and that, in fact, our brain is our ''soul'', for, our brain is what stores the vital information about our personality, makes us what we are. In a way, we are our body, but the brain is the part that is 'we' the most. Now, if there is no soul, then how could possibly a human (or an animal) become a ghost/spirit?
Another ridiculousness about this whole idea is that someone, in their right mind (or whatever you call it when it applies to ghosts) would want to hang around after death, whilst being unable to have any sensory perceptions (no body = no sensory perception) is just plain dumb. Imagine your existence not seeing, not hearing, not feeling, not anything. A sort of misplaced existence for an undefined amount of time . . . Without orgasms.
Now, the only somehow reasonable explanation for this ghost phenomena that comes into mind is that creatures might leave some sort of informational residue even after ceasing to exist. However, if such residue is made, it has to be stored somehow, thus, I suppose even this assumption doesn't make sense.
The real ghosts, spirits, whatever you call them, that exist, are the ones in your head.