Sans certainly hasn't been complimented much in his life. Papyrus mostly showed his affection through lecturing. Otherwise, Sans knows that he's easy to overlook and has spent a great deal of time making himself even easier to overlook. S-4's effusive praise about how kind and smart and strong he is is about as much as Sans can handle on any given day.

But handsome?

...is this what blushing would feel like, if he were capable of it?

Sans ducks his head down even further, trying to keep his teeth tightly shut against the laughter bubbling up inside his skull. But his shoulders still shake faintly with it, and when he does speak, the amusement is evident in his voice even if the breathy exhales don't quite sound like laughter. "Stop it..."

An idea occurs. Then he looks up at Mettaton and leans forward a little, head slightly tilted, something almost mischievous in its glowing eyes. "No more than you."