But you never know when you're going to be in a city, go to bar, meet someone who says something that gives you an idea that makes you go, "Hey, we should try doing that." And when you do it, suddenly you have an opportunity you wouldn't have had if you lived in a distant exurb.

Cities open more of what Steven Berlin Johnson calls "the adjacent possible" in Where Good Ideas Come From. As I said above, if you're interested in knowledge spillover effects promoted by cities, see Edward Glaeser's The Triumph of the City.