Do you remember the beginning when jae shin promised an old man to lengthen his life span for him in the book of life after losing a bet with him?
Jae shin did lengthen his life span but in exchange he reduced the life span of the old man son and added it to the old man life span
That was the reason the man stabbed nok because his son died because of that and that was also the reason jae shin was sent to the human world in the form of a puppy and the only way to return to his human form was to win a human heart.
Do you get it now

can someone PLEASE enlighten me why that old good-for-nothing rotten man stabbed him?! im so fucking lost and no one in the comments is wondering the same thing I thought this was the season finale but this is definitely getting a reincarnation arc and i can already tell how it goes
ANYWAYS someone pls answer my initial confusion !!!