
After some thought I think Jaeshin just wants to be next to Nok all the time (at least till Nok's death). If he went back to the underworld he probably couldn't be with him during the day. Maybe there are some limits how often he could go to the human world - like just to feed himself with yin energy. So once a 3 days? And he doesn't want that. If "the things are letf the same for now" he can stay in the human world all the time.
And remember he's hated this dog form so much, doesn't mind it now;)

Aww! That's so true. I was wondering if he couldn't be out and about in his normal form during the day. Also, I did go back and read and the brothers were saying that Lee Nok is fond of the puppy form... So it makes sense. ٩(๑❛ᴗ❛๑)۶ they're so stinking cute!
I really wonder where this story is gonna go. The author said it's gonna have around 100 chapters.

I've forgotten to add that it's only a conjecture of his brothers. I'm not convinced Jaeshin would've stayed in the dog form just because Nok likes Geumdong - apparently he likes Jaeshin more now and there're more things they can do if Jaeshin is not a dogXD That's why/how I've thought about staying all the time together :3
Wait! He earned the love of Lee Nok... So why does he still turn into a puppy!???