Maybe it's no more than a simple case of social expectations. We don't expect people to start dancing while riding the train (or for that matter, while in the library). Similarly we'd be surprised, and probably bothered, if people at a dance suddenly sat on the dance floor and started reading. We can read a book more or less anywhere, with or without other people around us, but that's primarily because even in a public setting we view reading as a private, or at least an individual, activity. For the most part dancing is seen as a social activity. But the fact that it normally takes place in public doesn't mean that it's not expected to adhere to established norms. Shopping malls, and elevators, have piped music, but only very rarely will someone start dancing in them.