There are numerous causes of pathological jealousy. Personal causes usually involve low self-esteem or narcissistic entitlement. Somebody with low self-esteem will constantly wonder if they are able to maintain interest of particular sexual partners or if their sexual partners are interested in someone else. People with narcissistic entitlement tend to want all of the attention to themselves and they will react negatively in a jealous way if their partner talks to someone else or pays attention to someone else.
There are also relational issues involved in pathological jealousy. If the relationship is unstable with low commitment level, it will often invoke pathological jealousy in the other partner. Jealousy's not always a bad thing. Jealousy, as any feeling, has a protective component. It alerts us that something is wrong in the relationship, that perhaps we're taking our partner for granted or perhaps something needs to be changed about the relationship.
In that way, when the jealousy is reactive, when you're reacting to the way your partner behaves, it's a healthy and normal emotion. It alerts you that something needs to be done. Perhaps the issue needs to be discussed and some things need to be changed in the relationship.