Two important posts from Ezra Klein, one on the enormous environmental benefits of even modest increases in residential density, and one the enormous happiness benefits of shorter commutes. Shorter commutes are, of course, facilitated by greater levels of residential density.
What's particularly astounding about this stuff, in my view, is that fixing the problem would hardly require some totalitarian density police to come around and force us to all live closer together. Instead, the main step we would need to take would simply be to allow people to build more densely if they want to. As a secondary measure, scrapping or limiting the tax code's weird and destructive subsidy of big houses would do some good. Everything Ezra mentions aside, I would also note that it's my observation that people (at least in the heavily-populated bad weather regions around the great lakes and the northeast) seem to systematically overestimate the amount of time they're going to spend in their yard.