If it's of a purgatorial type, sometimes humans can help out. They were very, very naughty in life and ghosthood is their cosmic retribution.They were very, very naughty in life and fear crossing over to the Afterlife and facing possible cosmic retribution.This type is the original 'grateful dead'. The person who arranged for their burial was a beneficent stranger, and they must make return for this good deed.

Or maybe someone is mourning them too much, and as a consequence, they are bound to this world.

No funeral, no grave - they cannot rest without proper memorialization - or perhaps their graves have been moved or desecrated.

They feel someone they love can't make it without them, or needs protection. In paranormal fields, these are usually called residual hauntings. The ghost isn't even the person's soul, but just a spectral imprint left behind by the person's death that's gained a form of sentience.

The ghost suffered so much in life that the spirit was drawn to the place where the worst torment took place.

No alternate dimension, Higher Plane of Existence or anything - you just become a ghost when you die.

That's just how the Afterlife works in this universe.

They're aware they're dead and angry at living people because they are still alive.

This can lead to a Tomato in the Mirror shock or a Spirit Advisor.

The ghost hasn't yet figured out they're dead, or are so attached to what they did in life they are still doing it out of habit and/or affection.The ghost hangs around until this is done, and may or may not move on afterward. Unfinished Business: Something that was significant or important to the person they used to be when alive remains undone.They may explicitly say that they cannot rest easy in the graves until they are avenged. This one can also lead to ghosts becoming violent and angry if not avenged. Sometimes, this also comes with a Clear My Name sidebar. Avenge me! The ghost was killed through foul play, knows it, and wants the murder avenged.It all depends on what kind of story is being told. Some are friendly, some are neutral, and some are scary and vengeful. Ghosts in fiction are usually people who have died, but their spirits are still lingering around.

Ghosts unfinished business