Jealousy is a very real emotion.
Real emotions are what you technically want your character to have. They can't be so perfect that they never feel anything. Your characters need to be real, so real, in fact, that the reader will be able to feel a sort of friendship with your character.
Jealousy is one of thos e very real emotions that we feel all the time, even if we don't know we are. Maybe it's a member of the opposite sex who's with someone else and not you, or how sometimes it seems as if someone's stealing your friend.
There's this girl in my class, let's call her Katy. Everyone hates her except one of my bff's Regina. And, you know how this goes... Katy needs help in math and everything like that, so Reg helps her. I, on the other hand, am perfectly capable of math, thank you very much, and do not need any help.
Sometimes I think I should just pretend to be suckish in math so reg can help ME not Katy. But Regina's a really good friend, and I know that she'd never desert me.