My usual reply is Why?
That normally stumps the troglodytes.
I've always had serious bitchy resting face so I used to get this a lot when younger, however bearing in mind I was a Nurse 95% of the time I had very little reason to smile on on own time. I was Mary fricking sunshime while on duty (where appropriate).
Best/worst response to this shit was a punch in the face.
We had a porter who was constantly saying this to female patients/visitors, several complained, several of us complained on thier behalfs.
We got the 'we'll have a word' 'He's only being friendly' 'It's only banter' How I hate that word..............
The thing was if a woman didn't respond or respond negatively, there was always a muttered comment; usually along the line of fucking bitch/ miserable cunt.
Anyway he did his smile it might never happen to a woman coming out of the bereavement office, yes really, and when she didn't respond muttered the usual comment; however her husband was attempting to catch up with her and heard the comment.
He asked him to repeat what he'd said which he did with a laugh, I believe the exact words were 'I just told the miserable bitch to smile, women eh?'
Husband punched him twice before someone intervened.
Sad that it took violence for this micro aggression to be taken seriously.
But considering the circumstances he got away very very lightly