I was at a ren-fest years ago. I wasn’t in a particularly bad mood, but it was near the end of the day and I was tired. Some girl walking past looked at me, flashed me a big grin and just said “Smile!”
Unlike everybody else in this thread, I actually smiled. It’s possible that hearing a stranger tell you to smile is easier to handle when she’s really cute and wearing a ren-fest corset.
My face is more super-villain than super-hero no matter who I am inside or what mood I’m in. I live with it so I’m used to it. If the person knows me and I actually am being grumpy, fine.
But to walk up to me and say ‘hey, your smile is slipping’ is really just them reacting to my face.
Personally, I’ll give them a glance while assessing whether they meant to be nice or to be an ass. If they meant to be nice, I’ll do my best to bring the expected smile to my face.
Sadly, some people find that rather intimidating/unsettling too.
Well, we can’t all be GQ models…
Happens to me a lot, too, and it’s just the way my face looks at rest. But I’ve received the “smile” comment a few times when I was really in a bad mood. Usually I just turn away from them but a couple times I’ve let them know why I ain’t gonna fucking smile at their command. I’m hoping the true responses that my mom was just diagnosed with Alzheimer’s or my dad just died cured those idiots of ever again telling a stranger to smile.
I’ve tried the true comment “I have a medical condition and I’m in severe pain” but that usually gets “you should see a doctor” or “take some ibuprofen/Tylenol/some other weak OTC pain killer” like I never tried any of those things.
This actually did make me laugh out loud. 
Like everyone else with a naturally non-smiley face, I used to get this a lot, and it has dropped off as I’ve gotten older, too. I can’t remember the last time it happened, now - I probably shot the offender a Death Glare or something.
I was ordered to smile recently by a US customs & immigration officer, of all people. His first words, when I stopped at his booth, were not “Let me see your passport.” or “What is your citizenship?”, but “SMILE!!!”
Not wanting to display a bad attitude and risk being questioned or searched, I complied.
Being told to smile by a random stranger is annoying enough, but coming from someone who has the prerogative to strip search you, it almost feels like an abuse of power.
I get it all the time, and it never ceases to be patronizing.
That happened to me, too. After I’d just been on a plane for 11+ hours with hardly any sleep.
I’ve gotten this on occasion. I tell them “I need the exercise.”
Good point, all those smiley people are just lazy.:Dlazy 
working hard
Yes, in 1972.
A good friend told me about the horrible thing that happened to her years ago, when she was a young woman. She had just flown in to her home state, and was in still at the airport, trying to get to the beside of her dying mother. She had just hung up with the hospital and received the news that she was too late. Her mother had died.
Devastated, she was standing there in the airport trying to decide what on earth to do and some asshole said, “Hey there, smile! It’s not that bad!”
Advice: If you want to speak to a glum-looking stranger to cheer them up, please pick something else to say.
Previous thread on the subject (that I didn’t actually expect anyone else to find; just thought y’all’d be interested):
Today someone asked me why I was mad.
I said, “I’m not mad. It’s just gravity.”
When they looked puzzled, I looked at them and pulled my face down with my fingers.
He laughed!
I used to get it occasionally. I don’t comply.
No one has ever said that to me.
You people need to smile more.
I keed, I keed.
The only two times people have said it to me have been in France. One (maybe drunk) guy at a neighborhood bar I used to go to often said, “You look like your wife died” – I grumbled something in French, I can’t remember, but either the owner (who I knew slightly) or an older drunk who used to hang out there and we’d often chat (can’t remember which) rebuked him sharply, for some reason. Some other time, on Bastille day, with lots of punks throwing firecrackers at people and stuff, someone said as I was coming home from work getting off the subway said something like “It can’t be that bad – relax!”
I don’t think men (I’m one) get it all that much as chicks.
I get it all the time, and I am a guy. It is incredibly obnoxious because more often than not, especially when I was working retail, it was meant as a power play or extra annoyance more than someone genuinely trying to cheer me up.
It’s also obnoxious because that’s just how my neutral face looks. I don’t smile unless there is a reason, I’m not an idiot.
I actually don’t get it all that often, but the most annoying instance like it was a dude passing me on the street. He said “hi, beautiful.” I said “hi” back, but was caught off-guard, so just kept my neutral expression. He calls back “why do you gotta look so mean??”
Maybe because this is what my face naturally does in bright sunlight, asshole.
For fuck’s sake, people, there IS such a thing as a woman with a neutral facial expression.
I don’t have a downturned mouth, but sometimes I’ll just plain be in a bad mood. And I have every right to be in a bad mood if I want to be.
What makes strangers think they have the right to dictate what kind of mood another person is in? I’m not bothering or hurting anyone, I’m just frowning. It just smacks of a sort of smug, condescending attitude from these people. It’s still a problem because I’m small with a baby face. Sadly, I’ve never had a clever retort handy because I’m usually just staring at them, shocked at their gall.
That’s how I take it as well. A power play. I always suspect that when a guy (and it’s always guys telling me this) it’s his way of asserting his will over mine, so he can get the tiny thrill of making me do something on his say so. If I crack a smile for him, he becomes pleased with himself, and not because he cheered me up but because I complied with his demand. It’s really creepy when you think about it.
This is why I ignore them and keep my face exactly as I want it. Sometimes I want to to tell them they are being rude, but I think it’s highly likely that they either won’t get it or they’ll just get defensive and dismiss me as an angry bitter black woman.