Yo. My house is haunted, no joke. My parents and my brother are the only people actually affected by it. Whatever "it" is in this house knows better then to even attempt to mess with me.

I think you left out one. Some ghosts, from what I've heard and if you want to believe it, just don't know they are dead.

I remember hearing a tale of a ghost that brought an armfull of (etheral) wood to a house every night. It seemed he was a ghost of someone who died in the early 1800's. He had been chopping wood hit his leg with the axe, and bled out before anyone could find him. That was the story. Thought I'd add that to the discussion.

People are likely to become ghosts after their death when

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They have many unfulfilled desires.
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Many personality defects, such as anger, fear, greed, etc.
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A lot of negative impressions in the mind.
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A high amount of ego.
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They have harmed others and have the basic nature of harming others.
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They do not exist in the positive planes of the universe i.e. Heaven and above.
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They have unfulfilled desires such as cravings for sex, alcohol (things that they can only experience through a physical body), revenge etc.
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They derive pleasure out of exerting control over and tormenting humans and other subtle bodies. Their general aim is aligned with bringing about unrighteousness in society.

The subtle body of a person after physical death is defined as a ghost if their characteristics and intentions match the above. There is no special process as such which they go through to become a ghost.

Ghosts aren't real.

Ghosts aren't real.