Love is a very irrational thing, and since jealously is as a result of love, it only makes sense that the behavior surrounding jealousy can be extremely irrational as well. Jealousy is kind of like a very angry, possessive form of love. It never really shows itself in attractive ways, and the word itself has very negative connotations.
Lets say you meet the partner of your dreams. You like them, they like you - and it all goes rather swimmingly for a while. You're at a friend's party, and the two of you meet someone else. You perceive mutual attraction between your partner and your new acquaintance. You start comparing yourself to your new rival. He only has one eye? You're alright. But its a pirate party! - now you're in trouble. Maybe she could be a part-time model? Perhaps you should intervene and suggest a trip to the drinks section to get the fuck out of there. You've already seen her on Victoria's Secret catalogues? Maybe screw the jealousy and suggest a threesome. She's hot and you'd totally do her. Plus, he's totally got no chance with her afterwards.
One part of being jealous about others is how your potential rival is perceived. You'd be more scared than jealous of the creepy guy making dodgy movements all night. Another part is how comfortable are you in the current relationship. If you never want to give them up, and would totally die if anything happened to them, then somehow everything is perceived as a threat. If you're a little more jaded, some might say realistic, and realise that they're here with you tonight, then nothing is changing that.
The other big thing is what you think of your partner. Are they the type of person who would dump you for something 'better'? Maybe you should be re-thinking about seeing them instead of being jealous all the time. Or maybe they're already batting above their average. You're safe until you take them for granted, but that's a whole other story.
