This is a common shibboleth among the self-help crowd. The implication is that smiling is more natural than frowning, i.e. you are really happy, you just don't know it.

The fact is, a typical sad person (anyone I've seen anyway) doesn't frown when he/she is sad or depressed. Remember, a technical 'frown' is an expression where the corners of the mouth are turned down, the symmetrical opposite of a smile. When a person is sad, unless he/she is talking or screaming or crying, his/her mouth is typically oriented neutrally, as a straight line. And this position requires the fewest muscles of all!

Anyway, it definitely takes more muscles to fake than it does not to.