If you smile when you are alone, then you really mean it.

The line proposes a simple test for sincerity: expressions performed without an audience are least likely to be strategic. A smile in solitude suggests an emotion that arises internally rather than as social signaling—approval, amusement, affection, or relief that does not depend on being seen. Implicitly, it contrasts private feeling with public performance, where smiles can be polite, defensive, or manipulative. As a piece of folk-psychology, it elevates the private self as a truer measure of meaning than outward display, while also hinting at the human tendency to curate emotions for others. The aphorism’s appeal lies in its clarity and its invitation to self-scrutiny.