It causes unhappiness because it highlights that you are relatively worse off. And I wasn't talking about comparisons between yourself and some random celebrity

It causes unhappiness because it highlights that you are relatively worse off. And I wasn't talking about comparisons between yourself and some random celebrity, but moreso a comparison between yourself and a person you might have been at college with. If that person is doing exceptionally well, and you're doing just okay, there is a strong chance it will stir up jealously and negativity rather than admiration. This is also the topic of multiple sitcoms, which generally tend to zoom in on relatable facets of human nature ;)