Ghosts, my entirely unscientific opinion.

I'll preface this by saying that I haven't researched any of this and wouldn't be in the least bit surprised if none of it was at all verifiable. I will write as if these things are fact simply because that is the easiest way to write about the subject. Prefacing everything with "I believe", and "in theory" would get repetitive.

To begin, I want to state that ghosts as I define them differ greatly from what is generally accepted. A ghost (which is an ineffective term to say the least) is in the simplest terms energy manifesting itself in a pattern that mimics past events.

What causes energy to manifest itself this way is not necessarily, or even frequently a result only of death. Often times a death is not very energetic (IE death in sleep). What would cause energy to build up is the repeated energy released by a family living in a house. More likely than just an average family to do this is a family that released larger amounts of negative energy. Perhaps an abusive household. Or a household grieving due to the death of a loved one.

The result is a buildup of energy in a particular area. This would account for the frequency of haunted houses, and even more frequent haunted hotels. Any area that receives intense amounts of, for lack of a better term, "psychic energy". The places that would best contain this energy are places that receive constant or at least rhythmic energy flows. Here are some of the places I would expect to see a "ghost": Abusive households, Sporting arenas, any bed, even a bedroom, cemeteries (due to families, not the dead), hotels, large terminals (Grand Central, LAX, etc.), places of slaughter, war grounds, prisons. Basically anywhere where a lot of people think and feel a lot.

An interesting side effect of this theory is the possibility of a self-haunting. A child who lives in a bedroom up until they move out may encounter a ghost of themselves. Due to the excessive amount of time spent dreaming, thinking, crying and so forth, there might be a buildup in the room. The result could be any number of things. Objects moving, air temperature, the feeling of presence, or any of the other things reported.

Another facet of this theory is that ghosts would most likely manifest themselves when large numbers of people are around. In fact, in large areas, many ghosts could be operating in their various loops and no one would notice. The only reason they are reported so often by individuals who are alone is that in the rare occasion a ghost manifests itself when there aren't many individuals to spark them, the lone individual is more prone to notice it.

The buildup of energy would behave similarly to a static charge. The energy is slowly increased. If left alone for too long the energy will decrease. But rhythmic or continued application of energy gets to a point where it will "spark". And whatever kind of energy has been building manifests.

An unexplored section of this theory may say that sudden feelings of euphoria or sadness may be a result of these, again for lack of a better word, "psychic discharges".