Why does paranormal activity happen?

Why does paranormal activity happen?
Paranormal activity usually happens for several reasons. 

-If a spirit is angry or has a mental grudge with the place of death it will stay there. 

-The spirit, unhappy with the way it died is staying there. But unlike my first reason, this type of spirit is nonaggressive. in pain 

-This spirits playful and doesn't want to leave. 

-It may not be a spirit, it may be a demon in which you have unknowingly invited into an area. Usually through messing with spirits or constantly trying to tap into the spiritual world. 

-Triumph, a ghost coming back to their place of their greatest achievement. 

-And finally, the spirit may not know its dead, if its death was virtually quick, untimely or shocking, the ghost may not think its dead and go on with its life as if it was still living

from here