What makes a flash mob successful is the fact that it's unexpected, and perhaps more than anything else what we don't expect is that people in generally atomized situations such as a shopping mall suddenly have something in common. Yes, they're concentrated in the same place, and they're there in order to buy (or window shop) but there's not really anything that brings them together. And what's pleasing to us is the fact that for a few moments they really do have something in common, something that, all of a sudden, binds them.