So keith is sad because he cant mark someone else, but he is not sad with the fact that ye...

Really? I didn't find out..

Keith is sad that he can’t mark yeonwoo (I think, coz he tried to mark him before I don’t think he’s gonna mark somebody else other than yeonwoo if he can anyway), and upset mad about the fact that yeonwoo can mark someone else other than him.

I feel the same way because the feeling in his eyes was like the fear of losing Yeonwoo and the fact that yeonwoo is no longer interested in him, however, both of them are hiding their true feelings.