Small courtesies seem to mean more in the city than elsewhere, maybe because so many opportunities for them are missed and because they are so little expected. In cities there is a sense one must aggressively pursue one's own interest as there seems a scarcity of time and space, or at least a frustration at having to share so much of it with people who are much less like you than they would be in small towns. But this frustration is also the source of the city's sublimity: this ability to connect, however ephemerally, with total strangers, with people whose lives you can't imagine, is a chance to enlarge youself, to be without a social horizon. (Of course some prefer social horizons, some like the idea that they'll never meet a person who can teach them anything).
The courtesies I am thinking of hinge upon a moment of hesitation, the extra moment someone waits to hold the iron gate open when leaving the subway stop, or the moment one waits to let you off the elevator. It takes much less time for the hesitation to register and surprise others, and usually they let you know its been recognized with a note of sincerity in their thanks that is impossible to miss. It takes just the slightest pause to make someone else think, oh, you didn't have to do that, thanks. This moment of waiting becomes a leisurely moment snatched from a day of compulsory hurriedness. Its a moment where you assert the priorities of social interaction above those of business, and perhaps because that itself is so rare that these moments feel more precious.