Have you ever been jealous of anyone ? Don't lie. You can lie to me, lie to everyone but you can't lie to yourself. I'm pretty everyone have already been jealous of someone.
To my mind, to be jealous is when you feel like someone else is getting/given something you want, need or think that you deserve. Most of the time, it's love, attention, affection or even a material thing. You can also be jealous when someone is too close of someone you love.
Life lesson : Being jealous is something that happen to many people but that's not a good thing. So instead of letting jealousy destroy you, try to understand where it comes from and why and then express your feeling.