The modern world forces (and enables) strangely intimate situations. Crowded city blocks jamming apartments into every square foot, overflowing sidewalks in major metropolitan areas, subways packed like tins of sardines and don't get me started on elevators.
Make a list of times or situatons you might be a bit too close to one or more folks. Pretty much any form of public transport will do, but don't forget things which enable a kind of mindless intimacy with strangers, too. Online chat services, computer games, 4chan (??), facebook, myspace (really? is it still around?), plus anything putting you into forced proximity with people (concerts, public restrooms at concerts, traffic jams even).
Explore. See what it does, not only to you, but the person or people around you. What has gotten you to this strangley familiar distance? How does it affect you as a whole?
Visit Ellen Bass, Sleeping Next to the Man on the Plane, on the Writer's Almanac. Get to work.