I think most smiles are fake nowadays and that's why we notice the beauty in the real ones more and more.
Perhaps it's the "fake it 'til you make it" theory where we push ourselves to smile and then eventually smile naturally and genuinely due to habit. I guess the point is to convince yourself that you are happy, therefore you start believing it & become happy; and only then show true smiles!
Some people do it out of politeness, i.e. when someone has just said a joke that really isn't funny so you smile just to make them feel better.
Then again, some people also just seem like they put on a fake smile just to spite or mock others.
Either way, it doesn't really matter because nothing beats heartfelt genuine smiles and laughter, which also happen to be contagious. ;)