Riding public transit has its ups and downs.
Lately, there have been a lot of downs in that the Metro train keeps having breakdowns and delays while I'm on it. There is nothing like sitting in a crowded cattle car with inadequate ventilation with a hundred of your neighbors on a moist and muggy DC day to make you appreciate your own autonomy.
Generally there is an unspoken etiquette to riding public transit, and generally it is followed. Heck, there's even an etiquette to riding in elevators, which pretty much involves not looking at the next person and pretending not to notice any funny smells.
Then there are the outliers. People who either just don't get it, or just don't care. These are the people who force you to listen to their too-loud music, strike up personal conversations with strangers, talk loudly on their cell phones and the like.
Like those two post-grads on the train today. I now know more about the angst in their social lives than I care to. Weighty topics like "Will we still be friends with Jennifer a few years from now?", "College was hard, but not that hard", "Ever since Will broke up with Sarah, he's just been partying too much", and "I want to go to New York to see all my friends, but I'm afraid my family will monopolize my time" are just things I could go on not knowing and still live a perfectly fulfilled life.
There is a quirky social rule that applies: The higher the population density, the more private people (should) become.
That or this one: The less personal space you have, the more jealously you will guard it.