There is always some form of jealousy in any relationship and its how one handles that jealousy that ruins a relationship. A person who is extremely jealous has an uhealthy view of the relationship and will eventually destroy it. A little jealousy is ok as long as it is expressed in a healthy way like saying" it hurts me when you do...." or " I don't feel comfortable when you talk to ....." Expressing jealousy can be good but it can also be bad. Jealousy really has no purpose in a relationship except to undermine it. If one is jealous about something in the relationship it is best to get it out quickly and deal with it before it festers into unhealthy emotions.