"Bloody hell, cheer up love - might never happen"
"Actually it just has, my grandfather died this morning"
"Jeez, no need to be like that!"
"Like what, responding to your unsolicited and unwanted "suggestion" with a fact as to why it's wrong?"
"I just meant that you should smile"
"Why? Because you say so?"
"Jesus I wish I'd never bothered, fucking bitch"
"At least there's something we both agree on, now fuck off and stop telling people what to do with their bodies."
Actual conversation I had with a guy once when he decided that my face should look a certain way because of his whims.
If he had geniune concern for why I didn't look happy he had two more acceptable options - keep his opinions to himself or ask if I was alright out of genuine concern, The unacceptable action is to police someone else's expression because he felt like he had the authority to and then get upset because he got called on it.
Anyone defending the guy for his "harmless comment" needs to consider what message this sends out that a man feels he is so entitled to tell a woman what to do that he will speak to a stranger because he doesn't like her expression.