Everyone experiences jealousy now and then. Some only feel micro-spurts every so often. Chronic jealousy comes from insecurity and fear. Even extremely accomplished people can be insecure. They probably are not insecure about the area in which they are accomplished. There could be a psychological sphere in which they lack self confidence in school, in obtaining or maintaining a career, or in attracting a romantic partner, or in maintaining such a relationship, or even a friendship.
Thus, it’s possible to be multimillionaire in the tech industry, and to have never learned how to maintain a romantic relationship. This could be a source of pain and lack of confidence about romance and could elicit feelings of jealousy if such a person perceives a rebuff from the person of desire.
A person insecure about his or her career could feel jealous when a rival gets the coveted promotion. And yet, this same person could feel confident and secure in an educational setting.
Self confidence is not all or nothing. Like almost all phenomena, it lies on a continuum. Same goes for jealousy.
In a sentence: Sean became jealous when he saw Angie, who he had a big crush on, sharing a laugh with a guy he’d never seen before, so he decided to be passive and give her the cold shoulder until he got over it.
For her part, Angie decided that Sean was being an immature turd and just ignored him.
Yeah, this romance is going to die on the vine.