Jealousy

So since I'm not a jealous person by nature, I would really like someone to explain to me what the hell would cause a normal person to flip out over something simple.  I have my own hang ups about really various stuff, but you can't try and control someones every actions and imagine that they'll respect you.  I'm insecure, but being an asshole just because of that insecurity has never crossed my mind.  I just don't get it, and I see it all the time.  So please, someone please explain...