zoomed http://zoom.it/9qx1
It is a lot easier to move around and work on one side of the border or the other, which leads to people naturally not meeting people on the other side.
In the east there are so many more people and there is so much cross border traffic due to trade (particularly from the auto industry) that a lot more cross border connections exist.
http://en.wikipedia.org/wiki/Stanstead,_Quebec#Geography
Check out the library:
http://en.wikipedia.org/wiki/Haskell_Free_Library_and_Opera_...
http://www.time.com/time/printout/0,8816,948520,00.html
As for the west coast, let's not forget about the geographical anomaly known as "Point Roberts":