I think jealous is one of the sign that you really love the person, because if you get jealous to someone then it means that you became affected. It also emphasizes that we don't trust that person 'coz we don't have the confidence to ourselves.

Jealous appears because we don't want that certain person to be with someone else, and we are afraid of losing that person.

In addition, we became jealous because we have the fear if the person we loved prefers to be with someone else, and that really hurts right?,,