it not sudden tho. sieon does like namwoo and he does love him in his own way but since he has trauma bc he was abandoned before by someone he loved he now thinks all other ppl are eventually going to leave him so he doesnt act on those feelings anymore. thats what namwoo is now trying to do with him. to make him understand that he wont leave and hes also waiting for sieon himself to recognize his own feelings and act on them. which is what is starting to happen here. hope it makes sense.