Probably from when they first met, that meetup scene had a lot of chemistry imo! Likely stemmed from that and grew some more. Sometimes, if I don't understand a certain couples chemistry, I like to reread it and focus on the MLs POV. It definitely helps
From what I understand jins obsession is just possessiveness ... I forgot which chapter it was but it was recent update where Jin shot gunmu and they show a backstory where gunmu is like a puppy who keeps tailing on Jin , so jin is likely wants their relationship to be the same as before where gunmu will rely and only see Jin ..
Ngl I completely mixed up names lmao- i thought you were talking about the ML (really bad with names here lol)
Ah... So it stemmed more from Jin narcissistic self
I'm really curious what's with Jin's obsession with Gunmu, is it platonic? Is it now romantically charged? But that doesn't feel right, it's hard to pinpoint. Was it based on his own twisted idea/expectations of what family love should be?