There's something about him which forces people to confront their own mediocrity. I can't explain that, only the result, which is that he's something like a litmus test for insecurity. People who are confident in their skills and straightforward in their relationships love him to pieces. People who know in their hearts that they're faking, or who survive by manipulation, hate his guts out.
In both scenarios people's emotional responses betray something about themselves. You can use their reactions as reliable guides to their own characters. My experience over time is that this is remarkably accurate.