Grass is greener is a bit off.
Its more of a amplifier both ways, where the great becomes greater and the sub-par becomes plain bad.
As an example for the great becomes greater, i see DDG as the unquestionable best gk in the world, despite many or even most rating Neuer higher. I dont see enough of Neuer to warrant putting him above the greatness of our Donut.
Could be a mix of grass is greener and tribalism, but then again i also think Hazard is better than Dembele, Robben, Isco and a few others that some would question my sanity of, so im not sure its tribalism, but more the amplification of liking or disliking what you see often.