Within my grade at school, certain people seek out (I'm not sure if they do it consciously or unconsciously) the negative aspects of other people in the grade, without seeing any of their good qualities (which I believe they, like everyone, have). I was wondering why people do this, not only at school but in society in general? Why must so many people spend so much time (and I mean A LOT of time) focusing on such insignificant and often superficial aspects of people?

Not all "why" questions are philosophical and I think yours isn't really. It's more a question about human psychology. That said, you seem to run "negative" qualities of people together with "insignificant" ones. They're not the same of course: some negative attributes are very significant. But either way, we can ask why people tend not to focus on the deeper, positive values of others. Well, often they do! Why don't they always do it? Oh, I don't know if you'll find just one or two reasons. But here's one that functions sometimes: when you find something deeply positive about someone, you can't help but feel connected or attached to that person. Any such connection makes you vulnerable to pain, to loss. People try to protect themselves from painful emotions. And so they tend, at least at first, to keep some distance by keeping the superficial or perhaps even the negative in full view.

And here’s another way in which focusing on the negative traits of others serves a self-defensive function. If I notice how virtuous, intelligent, witty, and beautiful X is, then I might be forced to notice how my own traits pale in comparison. In contrast, if I notice the peccadillos of X, then I can take great satisfaction in the fact that I’m superior in at least these trivial respects.

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