I have a problem when people tell me I have a nice smile. It is not just the fact that I am weird about compliments,i.e, I'm flustered when I get a compliment. What I find ridiculous about complimenting someone on their smile is the sheer senselessness of the compliment.
My point is, a smile is a good thing. It is supposed to be good. Beauty is the nature of a smile. The purpose of a smile is to make a person's face lookable and charming.
So when someone says a particular person has a pretty or beautiful smile, it makes no sense. The purpose of us being able to smile itself is defeated in a sense! Also, it seems like one is insulting others' smiles by complimenting a particular person on her/his smile!
So you see, it is a useless compliment really. And something obvious. It is rhetoric. It needn't be said to be known or discovered!
And if we do start measuring the beauty of smiles it's time we start judging frowns too! Because if we are stating the obvious, we might as well state the obvious on everything!