A true genuine smile is a facial expression comprised by flexing the muscles of both ends off one's mouth while showing the front Teeth without vocalization. It makes use of the face muscles, including the laugh lines around your eyes :Even in fake smiles if engages all muscles.
Thus, when we feel great and happy a smile comes out naturally. It's an outward sign of joy, amusement, excitement or contentment. The image below shows some fun facts about smiling

Thus, the importance of smiles tends to :(1) Lift bad moods
(2)Reduce stress
(3)Smiling helps us bond with others.
As it is known our body Language can influence our emotions, thus, smiling goes a long way to it,as it makes one's feels better and relieved.
In summary, smiling changes the Brain chemistry. So it can be a big help to people who are dealing with depression and anxiety.
So why not TRY SMILING???