Laura knows that expression simply because she herself wears it so often.
Falling for Clint Barton has its own particular facial expression – fond and exasperated and slightly sappy all at once. Though to be fair, on Natasha it’s more exasperated than sappy.
Teen, polyamory (not a threesome). Because if I was Laura, I’d enjoy imagining Clint and Natasha having sex, too. *g* This is how I can live with the weird farm family thing.