1. It makes us seem trustworthy: We generally interpret a genuine smile to mean that this is someone who is honest and trustworthy. Those who smile are rated higher in generosity, in extraversion and in friendliness
2. If you smile when you get caught you’re more likely to get off: Somehow we think that those who smile are really nicer people so we tend to be willing to treat them leniently.
3. It eases embarrassment: If you do something stupid like slip on a banana, or trip and fall in the middle of a mall, people laugh with (not at) you if you laugh or smile. That is, it changes their reaction so they’re less likely to mock. 
4. If you smile with others when they share good news, you’re less likely to feel jealous or annoyed at them: Interestingly, even if we smile politely but we feel slightly annoyed, our emotion quickly changes and we feel happy ourselves. Somehow we feel much better for having chosen to be “nice”.
5. It can ease any feelings of distress or pain: Smiling stops us spiralling into negativity and eases our feelings of shock and distress - if we force ourselves to smile when something bad happens to us.
6. It can help with problem-solving: When we’re stressed or nervous our focus seems to narrow and it makes it harder to find answers or solutions. But when we smile, the tension eases and we think of more ideas.
7. It can increase your ability to make money: Those who smile at their colleagues and their customers are usually more successful and are frequently promoted.
8. Smile and the world smiles with you: If you smile at other people, they will often smile at you, and they’ll tend to see you in a positive way!