Whether or not it is true that the world will smile with you when you smile, a merry heart does affect you in many ways, even as the Scriptures declare:
but a broken spirit drieth the bones.
Even when a smile is not what a person feels like producing, turning to those things that God gives that gladden the heart does change how a person feels. Then, with a merry heart, the life that God gives is perceived for the goodness that is in connection with the God who gives it.
This is why Thanksgiving and Christmas tend to be welcomed often even by those who reject God, for even though they refuse the giver, His gifts nevertheless make His goodness manifest, even in hearts that are cold and gray through disbelief. How much more so for those who have had their hearts turned to the Source of all goodness!