If you are talking about North America, then maybe Mike is on the mark, however, I would attribute the automobile to the Suburban Revolution.
Cities have a long history. Yet, you should probably look to the history of human settlement patterns to get any understanding why humans thought it might be good to concentrate together in high densities. The first thing that comes to mind are the city-states of ancient Greece. Because of successful agricultural practices and intensive trade with nearby cultures, including increasing technological sophistication with bountiful natural resources, Greece was able to form, whether intentional or not, a class system. Farming provided more free time, and those that didn't farm, either were merchants, laborers, clerics, or artists. How these events coalesce into a government, arguably the first democracy, is a mystery to me. Somehow, cooperation to form a "society" is part of the formula. Also, their education system must of had some kind of influence.
But then those nasty Romans came in with their effective armies and assimilated the Grecian culture into their own.
Of course, this loose explanation has clear Western bias. I have no idea how urbanization occured in Asia. Spiro Kostov has a gerat book on the history of cities, entitled "The City Shaped." Great stuff! Get it if you don't already have it.