Here are a few reasons why this might happen:

Comparison with others. Sometimes we compare ourselves to other people, especially those who we think are more successful, attractive or interesting. This can lead to jealousy in the relationship, as we are afraid that our partner might turn their attention to these people.

Fear of loss. When we love someone, we are afraid of losing him. This fear can lead to jealousy, especially if there are changes in our lives such as moving, changing jobs, or other changes that may affect our relationships.

past injuries. If we have had bad experiences in a past relationship, such as being cheated on or betrayed, then this can lead to jealousy in a current relationship. We may be afraid that our partner will also deceive us or will not be true to his promises.

Inconsistency in expectations. If we and our partner have different ideas about what it means to be in a relationship, then this can lead to jealousy. For example, if one partner believes that friendships with the opposite sex are normal, while the other believes that it is inappropriate in a relationship, then this can cause jealousy.

Low self-esteem. Low self-esteem can lead to jealousy in a relationship. We may doubt our attractiveness and worth, so we are afraid that our partner will find someone more attractive or interesting.

Even in the healthiest of relationships, jealousy can eventually arise, but that doesn't mean it's doomed to failure.