I know I had night terrors and had irrational fear of dark closets....My mom led a good life so I don't know where that came from. Maybe in my past one of my ancestors died in the dark and I got his or her genes. Or maybe it's karmic, in my past life I was killed in a dark closet and that energy stuck with this mind continuum I call me...dunno but it's fun to think about! My first Buddhist teacher told me karma is like a snowball rolling down a hill, it gathers more mass and energy as it rolls and that explains karma and rebirth.
Lots of people are horribly afraid of snakes and spiders but have never been bitten!