Ladies! What is a good comeback when a man tells you that you should "smile"

Honestly I get this a lot, and actually find it quite offensive. As if I exist solely to entertain him in that moment by giving him the reaction that he requests. "You'd look prettier if you smiled."

Also what is he trying to achieve anyway?

Does he actually think that his request will be received positively? Is he just trying to piss me off? His end game here I find a bit confusing. But all the same I don't think it's his place to tell me what to do.

I admit, I have a bit of a resting bitch face. But I also deal with chronic pain and if I'm not paying attention, my face may reflect this.

But still, what if I had cancer? Or someone I loved, just passed away? If that were the case it would just make the guy look more like an asshole in my eyes.

Also the other thing about this that really sets me off, is that I have never heard a man say this to another man. I dare the next man to say this to another guy and see how that lands. The comment just seems very pointed.

Anyone have any good comebacks for this?

If they are safe for work, that would be even better!