I feel like I don't really have anything to contribute. I'm only 14, and I've never been in a relationship. But I see jealousy in other kids my age. Jealous of the cheerleader sort of thing. Whenever I see anyone with that kind of jealousy, it almost always seems to stem from feelings of inadequacy. "I want to be like her, because I'm not good enough." People are unhappy with how they are (or, more accurately, how they are in comparison to how they think people want them to be in order to be accepted), and when they see someone who is what they want to be, they resent that person for having what they want. It's so silly. If you're happy with yourself, you won't be jealous. If you're unhappy with yourself, then instead of wasting your time and energy resenting someone else for it, learn to like yourself how you are or change it. Don't like those extra squishy spots on your body? Go exercise! Don't like being lazy? Do something! You have the power to make your life how you want it, so there's no reason to be pining after someone else's life.
Jealousy is a waste of time. Get up off your ass and make your own happiness!