Have you ever noticed that some people don't smile with their eyes? But can laugh at everything you say? It's actually true, in psychology's expressive behavioral explanation, the smiles in this senario is called a "social smile". When you are truly happy, your eyes smile too (just like the cat above...just kidding, you know what I mean haha). I guess people can always fake this as well, so it's not really a reliable sign of sincerity.