So let's clear up once and for all what is at the heart of yewt dewshery:

Pride. The bad kind. You see, for yewts, it's not enough to be good: they need to be better than someone else. It's not enough to have something: they need to have more than someone else. They live for comparison. That's why they disappear when things are going poorly: they no longer shine by comparison, so there's no reason to be a fan.

They only come back when they are favorably comparable in some way. They don't actually care about their teams, they just care about being better than someone else. They're empty inside.