"Once Mine... Always."

Some people just don't want to let go of old things. At least that's how I viewed it. Some guys (it's not only guys) tend to get mad at a girl for having another guy. Why do guys get mad when they either didn't make a move or they left the girl?? I thought that once it was over, it was over but now it seems like every time you move on they try to pull you back in. So first point why do guys get jealous when they didn't make a move? Well the gents say that it was doubts and they missed their chance. "Maybe he wasn't too sure of himself, then he saw someone else step up and he punked out." Jealous because they punked out? Sounds like a personal problem to me. You just "got up the courage" and then you lost your chance you must have took to long. Of course that is not the problem for all. The other things is that once a dude has been in a relationship with you, you are off limits to everybody else even if it's his friends, family or people he doesn't even know. So basically "once mine... always."It usually happens like that even if they deny it and even if they have a new girlfriend or whatever. So that makes me wonder... when does he get over it? If he's still jealous even though he already has someone else then what does he need to get over it. I believe that is a more "out of sight, out of mind" thing. Although it doesn't always happen that way. You leave one person, find a new person, then you see your old flame moving on so you get jealous, then you get distracted and then you don't hear from nor think about your ex... right? Seems as if you are mad without a cause, or maybe I just don't get it...