Mathew Yglesias has it right when he observes that our built environment---which is dominated by single-family homes spaced far-apart on large lots---is not the result of some natural phenomenon or some inherent American preference for suburbia:
[T]o make a long story short, we have the built environment we have because of policy. The past half century or so has been dominated by rules about maximum lot occupancy and minimum lot size, parking requirements, and floor area ratio caps that were designed to produce something like the suburbs as we know them. Insofar as we keep those rules, the future will resemble the present. Insofar as we change them, things will change.