If you're jealous of someone, you want what they've got. This is one of the most basic human emotions, and it is not pretty. Don't be jealous of my good looks: you're cute too.
Being jealous is among the least attractive things you can be. The word jealous is actually derived from a Middle English word related to zealous, which means emotionally intense. It conveys a sense of emotional pain at someone else's good fortune. Jealousy is a close cousin of envy, but it can also mean "fiercely guarding" — as in "I am jealous of the little money I've managed to make, so I'm not likely to blow it on a cheap toy."