Relationship-threatening jealousy can happen for a variety of reasons. Insecurity is a major factor. Lifelong attachment issues can be another. For example, those who grew up with an insecure attachment to a primary caregiver that caused them to be highly anxious seek constant reassurance and closeness with a romantic partner, perhaps more closeness than is comfortable for the partner.
There are times when family-of-origin behavior patterns haunt current relationships. “My dad cheated on my mom constantly when I was growing up,”.
Sometimes jealousy can be a toxic byproduct of abusive behavior.
Jealousy can interfere with the trust that is so critical to a growing, enduring relationship.