I don't think a man can understand how infuriating it can be to be asked to smile when you're in the middle of something — like thinking, daydreaming or just zoning out. Why should I smile at you, I think, just because you asked me to. Shouldn't I smile when I feel like it? It's invasive, it's sexist (women are supposed to always be happy and pay attention to men), and basically, it's just annoying.
I tended to ignore them, which sometimes then made them angry. "Bitch," they'd say. Or maybe, "Why so unhappy?" Sigh. How do you react when a man tells you to smile?