passion

Humans are passionate creatures. Our passions drive us, gives us a sense of belonging, and unite us as few other things can. Still, there are only a couple of passions that have been constants down the ages, passions that people from every place and culture can agree on. Love is one, but another is that "those no-good bastards over there are trouble." Of course, we quibble endlessly over the exact definition of "those" -- every culture, pretty much, has had a different group in mind. But the singular fact of prejudice per se was as recognizable in Ancient Greece, Rome, and Samaria as it is now in modern Greece, Rome, and Arkansas.
Not only do we disagree over who Those Bastards Over There (TBOT) are, but also over why we hate them so much. Nose size has been cited in the past as a reason, as has intellectual capacity (too much or too little), and bad manners (eating without implements, eating with implements, etc). Hate may be a massively universal thing, but we are shockingly divided over why we do it. Personally, I blame TBOT.
Psychologists, though, (hate us or loathe us) aren’t as sanguine about not knowing, and have spent a great deal of time investigating prejudice in its many guises. They have come to two broad classes of answers: (a) Reasons we hate each other, and (b) Reasons we think we hate each other. There's not as much overlap between those two as you might hope.