I think it in part comes from being trained generation after generation that if I work harder than you, I deserve more than you. So if people want to make things more equal, I will have worked so hard for nothing and that makes me unhappy.

* Note: I used "I", but I don't personally feel this way. If I found out a fast food worker made more than me as an engineer, I wouldn't care. In some ways they work harder. Also, I don't think CEOs bring as much worth to a company as engineers do and they make way more, so...