Never trust people who smile constantly. They're either selling something or not very bright.

The line expresses a hard-boiled, cynical suspicion of performative friendliness. A “constant” smile is framed as a social tactic rather than a genuine emotional state: it may signal manipulation (“selling something”) or a lack of discernment (“not very bright”). The quote plays on the idea that authentic human affect is varied and context-sensitive; unbroken cheerfulness can read as inauthentic, predatory, or naïve. In a broader literary sense, it aligns with noir and tough-minded genre sensibilities in which charm is often a mask and survival depends on reading people skeptically rather than taking warmth at face value.