The smile on your face is there because you are happy? Are you always happy? Do you realize you are smiling or is it something that just comes naturally? When you meet someone for the first time, do you notice whether or not they are smiling?
Okay so it might look like a toothpaste commercial to some people, but so what? Isn't a happy appearance a better option than an unhappy one? I think when someone is smiling, it makes me smile, and not just on my face - I find myself smiling inside.
If you see someone today without a smile, why don't you give them one of yours?