You may want to look at The Evolution of Jealousy, a review of theories about the development of jealousy. It suggests that jealousy is pretty much the same for men and women (not related to differences in what makes a man jealous compare to what makes a woman jealous). It also theorizes that jealousy is an early warning system that alerts someone to the possibility that their partner may stray. This allows the person to take steps to head off the infidelity.
This theory says that jealousy has an evolutionary and a social component. Jealousy is experienced under differing circumstances in different cultures. So part of it may have to do with various culturally specific ways of displaying interest in someone else.
Morbid (seemingly completely irrational) jealousy, may be related to mental illness such as OCD. People constantly checking on their partners may not understand why they constantly check, and therefore assume it must have to do with something they picked up.
There are a variety of reasons why people may want to stay in stable relationships—and not all have to do with bringing up children. There is an emotional component, too. In order to maintain the emotional support of a partner, the jealous person wouldn’t want their partner to provide emotional support to anyone else. Interest in someone else threatens the emotional relationship, not just the financial relationship.
I kind of like the idea that jealousy is an early warning system that one’s partner is displaying an interest in someone outside the relationship. This interest could well threaten the existing relationship by causing the withdrawal of emotional and/or material support. Breaking up a relationship also has negative social consequences such as loss of status. There are a number of reasons why breaking up is not a good thing, and so jealousy serves as an early warning system that allows the jealous person to take steps to prevent the infidelity.