The title is really self-explanatory: I've always subconsciously loved this trope, I think. Like MLs who are rude to everyone and really gentle with the one they like (has to be FL, though, not every gf) or just a very visible change in them (doesn't matter what) when they're with FL vs. everyone else.
Examples: Reply 1997 (the biggest one tbh, it's even pointed out, "he only gets soft around you" etc), ummmm Love Rain, Giant, Healer, somewhat Dong Yi.