From Philip Greenspun's weblog a bit about Urban planning lessons from southern Maine.
The first example along a similar line that got my attention was the social "liveliness" in college dorms. For some reason there were dorms that were very "social" - where everyone in the dorm seemed to how pretty much everyone else. Other dorms (including the one I was in the first year) were relatively "dead". Now considering that the assignment of incoming college students to dorms was essentially random, why the difference?
Eventually I realised the difference between the "live" dorms and the "dead" dorms was the layout of the building. The dorms generally had one of two different layouts. In the "live" dorms there was one entry/exit point that was the easiest to use, and the entrance opened on a public room. In the "dead" dorms there were seven exits that were all equally practical, and the main public room was isolated.
A simple difference in physical layout made the "community" a much friendlier and nicer place to live.
At present I live in a newer more-or-less typical suburban community for southern California. The entire community was built in tracts over a span of several years. The published crime rate is very low (this particular area is just a bit out of the way). The public schools here are excellent, outranking even the nearby private schools (another topic). The people that live here are largely upper-middle class familes with kids in school. Folks are generally pleasant, but outside of our kids, there is almost no interaction between neighbors.
Walking down the block and around the area you will see the pattern. From the street what you mostly see is garage doors. The "living"areas are at the back of the house and completely isolated from the street. From the front you can see upper-floor bedroom windows (generally closed and shuttered). A carefully landscaped walkway reaches to the front door, usually with some small nearby window (also shuttered). Often the front door is slightly offset or obscured by plants/landscaping so as to be nearly invisible from the street.
Not exactly a formula for incidental social interaction.
Oddly enough this area is a bit better than some of the nearby alternatives. Walking around the community is relatively nice, and you occasionally (not often) run into people when out walking. There is a trailhead into a local "wilderness area" within walking distance. Some of the other areas on the market when I was looking were positively unpleasant for walking. The notion of driving somewhere else to take a walk did not hold any appeal.
To a small extent the same "single exit" effect applies. There is a single intersection through which most of the traffic passes into the community. There is a single supermarket off that intersection that is simply the easiest to use. There is a single local elementary school which essentially all the kids in the community attend.
A great place for kids to grow up, but not an especially interesting community for a single adult.