So, after watching the eng subs (thank you @DEBb), I noticed there was this part where AY was saying how she was sorry because she knows his personality but still made him do things. And then during the part where she said "...I know your personality...", you could see Jjong smile slightly, like one side of his lips actually lifted. It was a really small action that you might miss if you didn't look properly. Maybe he was thinking "Yes, Ahyoung-ah, you know me, you understand me"....or maybe that's just my delusional mind talking haha