Why is he jealous?

Answer 1

Jealousy is usually a sign of distrust within a relationship. It can also be attributed to simple human nature. We have the instincts of a predatory animal, and jealousy can be an instinctual way of "marking one's territory."

With that being said, it all depends on who the jealousy is being directed towards.

If you (or she) is making it known to the object of your jealousy (another person usually) that she is your girlfriend or you are her boyfriend, then that is generally territorial and not as harmful as the other kind of jealousy- the type where you are only telling your girlfriend/boyfriend you are jealous and not marking your territory.

Jealously is a sign of insecurity as well, and usually spells out trouble in any relationship, especially if it is bad enough that you have to ask about it.

This is something that is generally understood, so when she gets jealous she is essentially saying that she does not trust you. When you return the jealousy, she understands that you are telling her you do not trust her, which she thinks is ridiculous because why wouldn't you?

What it comes down to is communication, there are deeper issues at work here than simple jealousy, and you need to sit down and communicate with each other what they are in order to move past the jealousy.

If this is not possible, than maybe you are not in the right relationship.