I just read that last night! Best response to that statement.
I feel like I'm missing something here. What is wrong about telling someone they look pretty when they smile? I'm going to go out on a leg and guess it is something random creeps tell girls who want them to ***** off based on context.
That's basically it. The scenario is usually that a complete stranger will walk up to a girl on the street, or often approach a waitress or someone else working in the service industry, and tell her to smile. To the guy, it might be seen as an attempt to strike up conversation with a girl he's attracted to, or maybe he thinks he's just being nice and cheering someone up. To the girl, though, it's intrusive, jarring, and makes it seem like she constantly has to smile on command, just to please random strangers. It's not as bad as catcalling, to me, but it's still invading a stranger's personal space for the guy's own pleasure.
My ex used to do this to women all the time. It was always a command, too. "Excuse me. Smile." He thought he was just being nice to those women. I couldn't articulate at the time why I didn't like him doing it, or why they would always scowl the moment he looked away.