When Lee Nok was a child in the afterworld, Jai chin told him “ it’s not your fault “.. and in panel 21 Kyung Joon said the same thing to Lee Nok, you can see Lee Nok remembering Jai Shin looking at his young friend saying “ it’s not your fault”.
So my theory is that Lee nok got emotionally attached to Jai shin since childhood ( even tho he forgot him ) and relatively got emotionally attached to Kyung Joon and fell in love with him

I just had reread the special chapter.. And realized in banner 21 that our Lee Nok might’ve fallen in love with Kyung joon because of the vague memory of Jai Shin!!!!! Omg. Omg!!!!!