I've talked a fair bit about this with my brother. I'm the urbanist nerd, and he's the PhD psychologist. The United States is a deeply traumatized society, which has a great deal to do with our history and hyper-individualist ideology, but it's now a feedback loop in which the hyper-individualist ideology perpetuates the trauma. Our built landscape is largely a redirection of that, so it's both a symptom and a cause.

You see how the two are wrapped up together by the number of people who say (often in this very community) that they hate being around people, that they couldn't live in a city, that they don't want neighbors, or at least want to keep them at figurative arm's distance, so they NEED cars. That's a manifestation of the chronic fight-flight-or-freeze response, because at an individual level, our mental model is of other people as competitors and possible existential threats. (What's wrong with public transit? CRiMinALs wIlL KiLl mE!)

Thing is, to we're a highly-social animal, so the more we pull away from other people in fear, the more stressed and traumatized we get. That is to say, that it's not simply that the built landscape causes loneliness, but loneliness and car-dependency are interlocked in a feedback loop with other factors.