It makes people defensive:

Whenever we talk about urban form, people hear us making judgments about their homes.  I can stand in front of a group of citizens and talk about how a certain kind of development pattern implies certain consequences for transit, and thus for sustainability, and thus for civilization.  As we talk, it may appear that we’re having a thoughtful and educational discussion about good and bad design.  But some people in the audience have chosen to make their homes in the very development pattern that I’m describing, and to those people, I’m saying that their home is good or bad.  Once you hear that, you’re likely to have a strong emotional reaction that makes you deaf to rational argument.  On some level, consciously or unconsciously, you’re going to feel as though I’d walked into your own livingroom and told you that your decor is not just ugly but a threat to civilization. 

Strangely, Matt Feeney's thoughts on the new Arcade Fire album are tangentially related.