you can say a lot more than otherwise because, when you don't smile, you would be fearful of blurting out words that'd offend, and as a result, you say less. (Do you want to say less?) A lot of words you cannot use with a long face, but you can with a smiling face.
With a long face, you want your facial expression to share the burden with your mind, because your mind is not strong enough to shoulder the entire load since it is flooded with mood.
The mood takes away a lot of energy, much of it is wasted. When the mood is frenetic, one's face tends to be stiff to act as a balance. So work on your mood, much of it is a reflex engrained since childhood. Think of things as small and trivial. Then that saved energy can be used to maintain that smile.
People will share that smile with you.
