First: Our smile shows others people that we are friendly.
If a stranger approached you and gained eye contact with you and then treated you to a broad smile you will have a fairly good idea that they are being friendly and certainly mean you no harm.
By contrast we would find it far more difficult to offer trust to the stranger that wears a scowl or unfriendly face features.
Second: A smile can make people happy.
When someone treats you to a smile, even a stranger, you usually smile back. For that moment you enjoy a flash of positive communication without saying a word. As you walk through a crowed area such as a main street you can do this hundreds times in a very short period of time.
When you smile at someone who is not already smiling and they smile back you have brought a moment of happiness into their lives which, who knows, could last all day.
Third: Smiling is infectious.
When you spend a lot in the company of someone who smiles a lot you will soon discover that their smile starts to wear off on you. It is hard not to return a smile; most of us do it at a subliminal level without thinking.
Fourth: Smiling can...