One of my favorite games on the Metro is to try to figure out how people traveling together are related. Sometimes it is pretty obvious they are family or a couple but sometimes it takes a little more observation and analyzing before you can make a speculation. It is interesting though how quickly we can process information about people just by looking at them, hearing a few snippets of conversation and observing their body language.
You can speculate whether the two people are good work friends or just engaging in the awkward coworker conversation of danget we were walking to the metro at the same time and its strange not to talk. Whether it is an old friend visiting from out of town or a new friendship still developing. If it is the weird first or second date or the comfort of a long term relationship. And sometimes I just don't have enough time or clues to really make any sort of good observation at all. But I enjoy catching glimpses of people's lives and how they relate to each other.