Millie is kind and open-minded and caring, but she's not going to take your shit.
Loona, of course, has little self-control and is remembering the whole "actions have consequences" thing.
I just wanted to capture that moment in relationships when you realized that you just pissed off your partner and you have that sinking feeling in your stomach when you're not sure what to do. Loona, especially, has always been bad at communicating or expressing her thoughts into words, so this is just the natural growing pains of being in your first relationship.