Responses
minseok has a lot of inner turmoil and because yuna is too pure, too free, too happy for him he feels insecure and is more aware of how "horrible" he is. being with someone in a much "better" state than you are makes you more aware of your situation... and because you are insecure (and have low self esteem) then you'll end up thinking that person is better of with someone else~ cue: "how can you love someone like me" or "i dont deserve you"
So basicly, Minseok rejected her because she make him happy? Seems legit...