What makes a girl so jealous?

When guys are already in a relationship they are more attractive to some girls. So we know that there are dozens of girls that will try to steal you away.

We see your eyes wandering and we hear you talk to your friends. We know you think about other girls and we hate it because we want to be your number one priority. Girls who are treated like a number one are less likely to be jealous and controlling. (Or girls who are more self confident and don't need all that attention to make her feel good).