I Don't Like To Share

We all have that person (sometimes persons) who we're really attached to. As a friend, as someone we care about, sometimes we see them romantically, sometimes not. It's that person we're very protective of, or better yet, possessive of. We consider them all our own and if anyone comes to threaten that by taking away their attention or presence we get upset. Maybe even jealous. It's someone we don't necessarily want to "be with" but we want to possess. We don't want them, but we don't want anyone else to have them.
Maybe there is an underlying attraction to this person that is unrecognized. If there wasn't, then we wouldn't feel so possessive and jealous when someone else steps in to threaten the relationship. I know someone who gets really possessive of me when I don't pay him enough attention. We're friends. Nothing more. But there is a change in his behavior towards me. He gets distant and snappy because his feelings are hurt. Whether or not he has any right to be is not the issue. It's just a feeling, and with most feelings, they cannot be controlled. They merely exist. The problem is that he doesn't want me but he doesn't want anyone else to have me either. Which is unfair for everyone involved.
I have a person or two that I'm really protective of. Who I feel I want to shelter from the possibility of having someone else get all their attention. We do that when people get new boyfriends/girlfriends. Sometimes we become jealous of the new person because we feel threatened.
Even though it's hard to tell why exactly we feel this way, we have to at least try to identify the reasoning behind it. Maybe it's because we're subconsciously trying to trick ourselves into believing that we only see the person as playing one role in our lives, when in fact, our brain is trying to tell us something different from the deep, dark depths of our head. I don't have this answer, and I wish I could control some of the feelings I have because sometimes, with the less than pleasant feelings, it just creates a giant circle that you have to travel on to absolutely no where. I'm tired of going no where. I need someone to give me some answers.