Jealousy is something everyone deals with at some point in their lives. It is something that seems to be a learned response that we develop during our growing up. It is wanting something someone else has or a feeling of missing out on something others have. Coping with jealousy is something we need to learn to handle on a case by case basis.
In a relationship, trusting your partner who may have cheated on you may bring on jealousy because of issues with your self confidence in yourself. You find you do not have feelings of jealousy with one partner because of your comfort with the relationship. With someone else with whom the relationship is not strong the feeling of jealousy may be present when your partner is around other people. You may feel like somebody will make a move on them.
Having some jealousy in any relationship is normal and healthy but if you take it too far it can create un-needed tension. If you are afraid of talking to anyone of the opposite sex for fear of anger from your partner, this would be an un-healthy level of jealousy to have to deal with. If you are being overly jealous for unfounded reasons you are actually breaking down your relationship instead of preserving it.
If you are the jealous type and have no proof that anything has ever happened to make you jealous, it would be best to just discuss these feelings with your partner. Bringing this out with an open discussion will help both of you deal with it together. Having a strong relationship with true love should never have any jealousy involved.