• Murder -- especially those that remain unsolved. Sometimes the ghost will not move on until the murder is solved. 
   
• Accidents -- Sudden death from accidents such as car wrecks, drownings, falls and fires. Hauntings are far more prevalent from accidental deaths than those caused naturally.
   
• Suicide -- The pain and torment that a suicide victim feels will often cause them to remain here in ghostly form.
   
• Broken hearts -- Much like suicide, the pain and torment felt by someone who dies while mourning a lost love or a family member may cause them to remain earthbound.
   
• Desecration of a grave -- While these spirits may have initially passed on, they will often return if vandalism, tombstone theft or other desecration of the grave occurs.
   
• Greed -- Sometimes humans carry a preoccupation with land or money which can create hauntings, when the ghost is unable to let go of its earthly possessions.
   
• Lack of a proper burial -- If the spirit believes he or she did not receive what they believe is a proper burial, they may remain.

It is under the small, dim, summer star.
I know not who these mute folk are
Who share the unlit place with me--
Those stones out under the low-limbed tree
Doubtless bear names that the mosses mar.

Robert Frost, The Ghost House