Aside from them being the main couple and the inevitability of it all, I thought the author made it pretty clear how Kim Dan fell for Jaekyung. Jaekyung comes in direct and gives him a job when he was one paycheck away from being homeless. He then saves him from getting r*p*d, gives him housing, feeds him and nurses him back to health. Then proceeds to pay his debt off and his grandma's hospital bills. Jaekyung even plays nice with his grandmother. Not to mention lets Kim Dan keep her wedding furniture in his house. And their physical intimacy has also increased even more so as well. Yes they didn't start off in an ideal circumstance and Jaekyung is an ah but he is Kim Dan's ah, lmfao.