What factors create a ghost?

Scarterran ghosts are like the ghosts of most folklore. They are the spirits of mortals who die with emotional ties due to unfinished business.   Not every mortal who dies with unfinished business becomes a ghost. They need to need to be lucky or unlucky depending on one’s view.   Most Scarterran souls pass on to the afterlife when their bodies die (becoming the honored dead). An extremely tainted soul will be drawn into Void and face utter oblivion or in rare cases, be transformed into a type of incorporeal undead.   Ghosts have enough a touch of the Void upon their souls that they do not pass into the realms of one of the Nine, but they don’t get so much taint of the Void sucks them in and obliterates them. This normally only happens when a soul dies with great unresolved passions or deep regrets.  

Why do some people who die with unresolved issues become ghosts and others do not?

  Mostly if the story needs a ghost, there’s a ghost, but because I want my world to be internally consistent there are noticeable tendencies. Ghosts don’t normally appear immediately, they tend to rise a few days to a few weeks after the body’s death.   -Only mortals can become ghosts. In this "mortal" refers to someone with a complete mortal soul. This leaves out Fair Folk, beasts, elementals, and spirits. While these non-mortal beings have mortality in that they can be killed, they do not have mortal souls and thus cannot linger as ghosts. They can still leave behind incoporeal undead remnants that may ressemble ghosts, but these undead entities are creatures of instinct lacking the free and passions that define a ghost.   -The number one factor in determining whether someone will linger as a ghost or not is how many regrets the person has when he or she dies.   -Strong willed people are more likely to linger as ghosts after they die than weak willed people.   -Violent and painful deaths are more likely to produce ghosts than peaceful natural deaths. Though slow, drawn out natural deaths due to disease produce their fair share of ghosts.   -Corpses that undergo proper funerary rites are less likely to produce ghosts than corpses that are left out to get picked over by scavengers.   -Divine spell-casters and pious people in general are less likely to linger as ghosts than areligious people. The Nine would like to see that no soul is missed, but they put forth more effort to collect the souls of their favorites.