system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly causes paranoia?

Why do we fixate over irrational things and what makes the body react the way it does when we do fixate... (ie. loss of sleep/appetite/concentration) 
Maybe if I knew what caused it, I could learn to switch off. 

EDIT - Before this goes all weed-based on my ass, that wasn't all I was going for. I meant paranoia in general.
gpt
Paranoia and Panic Attacks are reactions to stress or unbalanced brain chemistry.  
Essentially your body is reacting to stress like it did when our ancestors were living in trees and caves.  You get a dump of adrenalin and other hormones to help you focus and discern danger.  With no physical danger, those chemicals start your mind racing and you're trapped thinking about that guy who gave you the stink eye at the mall and rehashing every conversation blunder you've ever made.