You just wander up to the nearest available stranger and let the dialogue commence. If it turns out to be a monologue, or uninteresting, you excuse yourself and select the next nearest person to try it on. When I'm walking around, I'm pretty sure I look none too happy. It isn't something I have to think about to maintain. Possibly because a few ingredients of mine are Chinese genes and upbringing, I tend to not wander around smiling and gabbing like Mr. Rogers in the land of make believe.
People are a lot like daytime television in that respect.
Those people have rich, varied social lives and many good friends. The TV is like your friend that talks on and on without worrying about whether you're responding or not. People like people that blabber on and on non-stop. Today as band practice was cleaning up, I was listening to the people in the vicinity talking to each other. People say such stupid stuff just to say something .