"My group vs. theirs". Engineers vs. Physicists, Upper Class vs. Lower, White Skin vs. Green Skin, People who wear Green Boots vs. People who wear Black ones, North vs. South, Sword vs. Sorcery, Motorcycle vs. Car, City vs. Land, etc etc etc etc etc.
Be it profession, philosophy, religion (of OotS, of course), taste, birthmarks, ... you pick your choice and you will always find people who draw (sometimes arbitrary, sometimes silly) lines and borders and find reasons why this might be better than that. It's pretty universal behaviour and probably has to do with that "you can trust your tribe but not anyone from any other".