For most of the duration of the evolution of the human race, humans were living in groups of < 50 people. Cities of tens of thousands of people are a very recent phenomena. That said, evolution did not halt 5,000 years ago, and there probably are even more recent genetic selective pressure that has made it easier for people to live together in cities. But still, my broader point stands, a lot of rules and conditioning are needed for people to live together without driving each other crazy. Some of this conditioning we are not even conscious of, because the conditioning occurred when we are so young. But when you watch the parents of young children, you see how much effort it takes to do well.