It's always about how you're affecting them or their mood. If someone were really concerned about you, they'd say "are you okay?" or "is anything wrong? You seem quite down." When you say "you should smile more", it's as if you're affecting their mood, and they don't really care whether anything is actually bothering you, but just that you're bothering them. But I'm not sure how it is specifically sexist. I understand it's offensive, as I've explained earlier, and wouldn't like people even saying it to me. But since I've heard people saying that to either gender, I don't know how it's sexist when it comes to women, but not sexist if someone says it to another man, or a woman says it to a man. It just seems like something you shouldn't say to either gender. It's not as if it's more acceptable to say it to a guy somehow.