They both have changed a bit as it has been 10 years but I don’t think that’s the problem, since they’ve recently had ‘moments’(afkalskd sorry I don’t know how else I should describe it). The problem is he has lost her trust from bailing on her so many times. She’s scared of him doing it again (ever time he does it it makes her so sad,,,I hate it😩). He notices she putting some distance between each other. He doesn’t like that she’s distancing herself from him and that’s why he reminisces of the past. Showing their past also gives us backstory of their first interaction.
At least that’s what I like to think because he doesn’t seem like a bad guy, he genuinely looks like he likes her and we haven’t seen him with any other girl who he is affectionate with like ham na bom. Right now his family is really the only problem in which stops them from being together.