Man's inhumanity to man. Best way to understand this is if you watch documentaries on monkeys and if you believe we have evolved from them. Monkey clans don't typically fight other species but they are ruthless with rival groups. All for territorial, food and mating rights. We see differences in each other, be it skin color, language, cultural practices and worst of all ideas, as a threat to our own being. But if we share a hobby with someone from a denomination we don't like, we make an exception, saying my buddy X though, he is alright. Find common ground, do things together, break bread together and everyone will be 'alright'.