The sincere smile comes from within. I think it's rare if it comes easily to a person. But with some effort, the inner smile transmits to the recipients of the heart.
Old school reference: If you ever watched Ally McBeal, a character, John "the Biscuit" Cage practiced Smile Therapy. I never forgot about it, and a friend, F, reminded me of it recently after seeing an earlier blog post of mine about positivity.
Sometimes, you have to fake it 'til you make it in life. And this is an example of one of those times. If you are feeling crappy and in a bad mood, try to relocate your inner smile to your outside self. Start smiling to yourself. Seriously. Do it a few times for a minute or so. The same affective result could be achieved by calling a buddy and laughing about nothing/anything. What happens next? Your mood will be lighter. You will be able to smile easier. Try it!