[Evil Lisbeth]If a stranger or someone I don't have a personal relationship with told me to smile, I'd respond: "Give me a reason-don't tell me to smile."
Or I might say, "My mother was recently murdered-I don't feel like smiling" (even though in real life she's still alive) to such a rude person-they don't need to know the truth, and it might make them think twice about telling random strangers to smile or butting into their business.
Or if they make unsolicited commentary about my purchasing habits, I might respond, "Thanks for your unsolicited commentary" in my sweetest voice coupled with Miss Manners' Icy Glare. [/Evil Lisbeth]
Actually, I might just say to them, "I really don't appreciate being told to smile by people I don't have a personal relationship with-if you don't like my facial expression, find something else to interest you. Leave me alone."