Re: [AmE] is it rude to say "cheer up"?
Just to illustrate an actual story, I had an important meeting to attend one morning when I was going to fly out later for my grandmother's funeral later that afternoon. Not in the happiest of moods, naturally, I arrived for the meeting only to be met my a colleague who took one look at me "Oh, what a sad face! Cheer up, Barb! It can't be that bad."
I'm afraid that I responded with something like "Well, [name withheld], when I leave here, I'm getting on a plane to attend my grandmother's funeral, so please suggest what expression I should have on my face, if mine upsets you." I left her gaping like a fish and sat down.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.