jealousy

Why is it that in our human nature, we become very jealous. Just recently a friend of mine became very jealous of me for a very small issue, and it turned into something much larger just because she was jealous, and that seemed to control the whole thing. It makes me think back to the topic of free will, only because mabe we are just being told to be jealous as kids, or rather we see others being jealouse and so we feel that we should to, and it just ends up sticking with us. Just like free will, we are told about it, but i think that if we were never told about it and no one thought about it then we would not have it. However now that we do feel like we have free will and jealousy, it will always be there.