Because they have to have it acknowledged by EVERY person in the country that everything about them is the best. Best players, best manager, best fans, best songs, best banners, best team loved by the neutrals (the best neutrals, of course, not your bog standard neutral...no sireebob). They're scouring every forum in the land right now trying to evangelize amongst the natives. I can never remember United fans going to the trouble...or Arsenal or Chelsea for that matter.
But everything has to be the biggest and best - and agreed upon as being the biggest and best - with this lot. They're basically the Texans of English football.