A smile is taken as some type of agreement and possibly a commitment.
Try saying no to a child while smiling…they will keep saying ‘Oh pleeeease’ until you get cross (and stop smiling.) Say no to another drink while smiling and its ‘Oh go on.’ Try smiling while saying no to a salesmen. They will not believe that you are seriously refusing their amazing one-day-only offer until you stop smiling. Even then it will take longer to get rid of them because you smiled in the first place.

It invites people to share confidences.
It makes us look caring and approachable so that others start spilling their deepest darkest secrets; and we don’t want them to. People who say ‘people are always confiding in me’ bring it upon themselves by smiling too much.

A smile indicates to others that we are nice people.
Our innate sense of fair play makes us feel that we might be misrepresenting ourselvelves if we are not.

We don’t want to seem cheesy and insincere.
We smile at our friends but why bother with strangers when we may never meet again. We don’t tell everyone to ‘have a nice day either. This is because we don’t actually care what kind of day they have.

Lifting the corners of our mouth and baring our teeth seems a bit primitive.
Well apes and monkeys do it. Surely we have evolved.

Basically we aren’t that keen on other people we prefer our pets.
Pets don’t care if you smile at them or not.

We fear rejection.
If the other person does not smile back, we will feel stupid and as though they somehow have the upper hand. It’s far too big a risk.

Our teeth are too ugly.
Well there is a National shortage of dentists!