Sometimes I think we forget how to smile. I think we get so used to putting on that fake smile that we forget what it's like to actually smile. Between posing for facebook pics, plastering that fake smile for work, or when talking to that particularly annoying family member/friend. We forgot how to just genuinely smile.
However sometimes that real smile comes out, and usually it's accompanied by an unexpected event. Much like the ones these people experienced when they logged into chatroulette.com.