Jealousy

What is Jealousy ?

Jealousy typically refers to the thoughts, feelings, and behaviors that occur when a person believes a valued relationship is being threatened by a rival. This rival may or may not know that he or she is perceived as a threat.

It is important to distinguish jealousy from envy.
To oversimplify, jealousy is angry, envy is wistful.
Jealousy is destructive, envy rarely is.
Jealousy is Insecurity.
Insecurity means a lack of security. It does not imply that the insecure person is bad, or good.
It simply means that the person is not secure, either in themselves or their relationship.
e.g., is uncertain or in danger.

In some cases, the insecurity is not founded on realistic dangers to the relationship. If that is really the case, then you (the jealous partner) may wish to consider where your insecurities are coming from. Solving those sorts of insecurities isn't easy, but until you do you'll continue to face those feelings.
Since we tend to become more secure in relationships as they become more stable with time, time is your ally in dealing with jealousy.

Source - From Wikipedia, the free encyclopedia