Baudelaire's theme, or one of them. Of which my favourite poem is his sonnet "The Passerby," a classic anecdote about the person whose eye you catch in some public place, the person with whom you feel an immediate rapport, but whom you will never see again. In his case, and most often, in poetry, it is a man who sees a woman, This can be updated, not just the woman who sees the man, but any potential friendlover. When there is no erotic ping is perhaps the most moving: you have a sense of connecting with some other. It could be a dog you are about to euthanise...
But I was thinking about something else when I got onto this. How, when I walk down the Bd St Germain, among the crowds, I always find myself zigzagging to avoid the people walking against me. Why don't they zigzag, I think resentfully, why always me? So when they come on, I stop, I force them to flow around me.
But I have discovered the secret of walking in Paris crowds (does this happen in NYC, in London?): no one makes eye contact, but if you keep walking, at the last moment, everyone twitches their shoulder slightly, so that you pass without touching, though you feel the slight change in airflow.
I could compare this to crossing the street. In California, mostly the cars slow well before the crosswalk, if you are waiting to cross. Here, the cars stop--at the last possible second, and sometimes you need to look like you are going to walk right out in front of them: they aren't going to hit you, but if you have a heart attack, that's ok.