I watched an episode of BBC series "The Human Animal" named "The Human ZOO" last night. The author and presenter -- Desmond Morris talks about the our ancient habits and how they reflect in the modern life. We need to belong to a smaller tribe, and thus living in large cities is very unnatural. He points out how we develop our own tribes, that are formed by a group of people who surround us, our family, and friends. These people do not necessarily need to know each other, they just belong to your own unique tribe. Every person has her/his own tribe, and this is how we survive in the flood of people that live around us.
The author did an interesting experiment, where he would put a person on the ground and observe what others do about it. In the city, people look, maybe even slow down, but definitely keep walking past the possibly suffering individual that is laying motionless on the ground. In the village, however... the person is immediately surrounded by the locals (tourists still keep passing by the person as if it was a tree) and offered help.
That made me realize how vulnerable we really are. No one is watching over us when we walk around alone even during a daylight, since you are just another tree in the forest of people who everyone tries to avoid as they are making their way though crowded streets of the city.
Why do then people want to live in the cities? I think the answer is the number of options city can offer to you: choice of educational facilities, art museums, nightlife, shopping, dining... all of this makes us want to live in the city. But we loose the comfort of a tribe, and want to keep our borders and not allow the people who live across the street to enter our tribe.