Jealousy is an attitude in which we are on alert to prevent the loss of an object of love.
Jealousy is an emotion derived from expecting to feel sadness.
An older brother may pick on his new baby brother if he appears to be stealing away the mother's love. Or you may feel jealousy if you see your lover getting along well with another member of your gender. Jealousy occurs by expecting to lose an object of love.
Therefore, if you love your job you may feel jealous of a capable person who could potentially take your job from you.
When we feel jealousy we act to test that relationship in order to strengthen it. For example, a wife pinches her husband when he looks at an attractive women flirtatiously/longingly. It is a weak attack against the object of love. Jealousy is an emotion designed to maintain and affirm love. Jealousy can be called the defensive behavior of love.
Jealousy is an emotion derived from sadness. Jealousy is an emotion which combines the actions of anger with the danger of losing the object of love. So there is an inclination for those who easily become angry to easily become jealous.
Something similar to jealousy can be seen in chimpanzees. A chimpanzee male sometimes takes a female away from the group to mate, and if the female resists, the male repeatedly attacks and soothes. This is thought to be a prototype of jealousy and hate.