Smiling makes you happier:
Actions are followed by feelings and feelings are followed by actions. So when you smile, even when you are not happy, you give a signal to your brain that you are happy and this gives you a happy feeling.
Smiling helps people like you:
We like people who like us. When you smile more often, people receive this as being open to them and liking them. This makes people like you back, smile back at you and it improves the overall relationship.
Smile even when you don’t feel like it and soon you will feel like smiling more.