Jealousy can be fatal to relationships as it is poisonous. People usually say "a bit of jealousy is needed." I beg to differ. From a little bit can lead to irrational which then leads to something that can no longer be controlled.
Personally, I feel it's a lot about that individual's insecurities. Maybe, it's their nature to be that way. Therefore, before developing into something more serious, it's best that they overcome their own insecurities first. This actually is related to their own self-confidence and esteem. Questioning the partner will not work, controlling them will not benefit any of them either.
Possessiveness comes from jealousy too. It's all about self-control and self-reflection. Usually, these people will deny that they are feeling as such. Being jealous is never an attractive attribute. It mostly felt by girls in general. But, don't put off the guys as there are guys who are being like that too. The moment they feel that way and act up, argh, they become so unattractive. There may be reason to be that way, such as not losing someone you treasure but that does not mean that you need to be possessive. There are other positive ways to overcome it. Furthermore, being jealous is uncool. Get a life of your own is really important.
First, question yourself. Why did that other person choose you when there are so many other people to choose from? Personal space, people. The guys can be the girl's good friends, the girls can be the guy's good friends. Issue is, they chose you and if you wanna prove them wrong, please be jealous.
Of course, this is from a relationship point of view. Jealousy can be existed too in friendships or any of that sort. That is absolutely possible. Avoid feeling that way because to me, jealousy is a devil. It causes everything to be bad, to be bitter, to have ill feeling towards another person who is innocent. Considering the world now is already screwed up, is already very cruel, don't add on to that.
Perhaps, the quote "When someone is yours, it will be somehow." It may be a big round tour, it may not. But, if something is meant to be, it will find its way back to you. Guess, people need to learn to be more open with things and people :D