Jealousy is a deep, natural instinct. There are tons of studies that show how various kinds of monkeys/apes will happily eat a piece of fruit or whatever, but if you give the monkey next to them a piece of candy instead, the first monkey will go ape shit and never eat the fruit again.

Many, many people think like this. I remember talking to a software engineer from MIT a while back, I don't even remember what we were talking about. But, I distinctly remember him expressing an attitude where someone else received something that he felt that he deserved, regardless of whether the other person deserved it also or not, and to him, this was just about the gravest injustice that he could imagine. He was like visibly angry clenching his teeth while talking about it. And it had nothing to do with whatever the thing was, hence why I can't remember it. To him it was a very deeply rooted idea of "fairness" defined in this sort of simple-minded, inflexible way. And he was ALL ABOUT removing the thing from the other person as opposed to trying to get compensated equally himself.