Yeah wow now that you pointed it out, I wonder too. Hope the author would be able to explain this one. Because all they did was had sex, which Moks is pretty much used to....
Tbf, while it doesn't explain it, the author seems to imply that he has bpd and that helps contextualize things a bit.
tbh I don't know that much about BPD but rn it seems like he's only showing symptoms when it comes to Nanjo. i would've liked to see some symptoms in his other relationships too or even just have some mention of him getting obsessed with his exes. that way his behavior towards Nanjo would feel more natural
I'm no psychologist nor have any idea where the story is going, but my best guess is Mok saw how much Nanjo wanted to "protect" Sunhwa from Mok and fell for that part of his personality. He'd been given up by his biological parents and then essentially had the same happen again with his foster/adoptive parents and here was a person that could look at someone they thought needed help (Nanjo to Sunhwa) and are willing to do what it takes (his "relationship" with Mok) to do so.
Then, as Nanjo asked him any kind of personal question, Mok perceived it as interest in him and when Nanjo brought him coffee/breakfast, he perceived it as Nanjo caring about him. His previous partners and the friends that he's had for 10 years are mostly surface level from what seems like both sides. Mok doesn't open up to anyone else, so they can't know what he's thinking. They mainly hang out to party/drink or sleep together (minus the one that came to check on him, but that whole time Mok was completely tight lipped). Nanjo didn't have to be over the top special, Mok just felt like he was being seen and now doesn't want to give that up.
It's very sad and why I feel so bad for him and want to wrap him in blankets and snuggle him. I saw another comment mention the author saying Mok had BPD and it tracks. Nanjo is doing the right thing keeping him cut off (not sure why all the Nanjo hate, lol) but it really sucks when you feel this connection with someone and then ruin it with your own actions and perceptions and then want to make it right more than anything to bring that connection back even if it never existed in the first place. Mental illness will only exacerbate the issue.
Mokyeon once asked himself the same question:
"What are you, what's so special about you?
Haa... no.
What's special is how I feel about you."
source: https://www.mangago.me/read-manga/the_boundary_of_delusion/uu/to_chapter-21/pg-1/
Of course i agree with everything you mentioned above haha Nanjo is full of charm and qualities and those are particularly attractive to Mokyeon
i wonder why before meeting Nanjo Mokeyon doesn't seem to get that attached to people, but then Nanjo who he's not even that close to somehow triggers such an intense attachment? Mokeyon doesn't seem attached to his friends that he's had for ten years but then becomes that obsessed over a guy who doesn't even care about him. i'd love to know what it is about Nanjo that makes Mokeyon react so differently because almost killing yourself over a guy who you weren't even in a relationship with is insane behavior for someone who usually has no problem discarding people when he gets bored so clearly it means that there's something really special about Nanjo but i just don't understand what it is