In the end, Yukhei knows that Donghyuck held no romantic feelings for him, but he can't bring himself to let go of Donghyuck.  Or, Donghyuck is dating Mark, and Yukhei is just the fall back boy. 

Donghyuck falls head over heels in love with Mark, quite literally.

Mark never really understood what love was. He always heard tales about it, but never though he could experience it when he was diagnosed with a bone tumor. But that one night, that night he realized what love felt like. Love is so beautiful. Mark fell in love with Donghyuck with one kiss, but he'll wait forever for another one.