What do you mean by "bad"? If you're asking why it makes sense to structure society with an eye toward avoiding disparity, then it's enough to just observe empirically that people have an aversion to unfair treatment. And not just people: https://en.wikipedia.org/wiki/Social_inequity_aversion

If you're asking why do people respond the way they do to disparity, then I can only speculate that it has something to do with the meaning of life.