In an interesting post, Upzoning Manhattan, Matthew Yglesias asks, "Should Manhattan get even denser?" and answers "Yes". Cities are green, and denser cities are greener.
Despite the fact that the demand for increased density is ever-present in economically successful cities, people who already live there usually resist. This is certainly true here in Cambridge. As I've mentioned before, there is a struggle in my neighborhood to decrease the density of residential units being built on what was once a commercial greenhouse and a heating oil distributor. People are quite angry about the negative externalities of having more people in their neighborhood (traffic, mostly).
Pretty much everyone who lives here does so because of the density, which results in easy access to food, entertainment, specialty stores, and interesting neighbors. Those additional neighbors will help support more of all of these things. And if we don't provide places for new people to live, housing prices here will continue to rise, and they will move elsewhere, eventually to some other part of the country. We lose not only liveliness, but economic growth and political influence.
Current neighbors have a vote, potential neighbors don't. And let's not get into that annoying issue of private property, and the pesky rights that go along with it. The solution is always to rezone so that people who bought the property hoping to develop it under the old zoning no longer are able to do so. Eventually, no building will occur, and Cambridge will become a retirement community with universities and biotech firms in it. I don't see that as a viable future.