@Casanova haesung like euihyun but euihyun doesn't feel the same and he's feeling quilty for it because once he told taeju that "what if he likes someone like haesung" so what I gathered from that is the fact that he likes the idea of it (kind and caring partner)not haesung himself and this is why he's apologizing for taking advantage of his kindness.
THE D1LDO OF CONSEQUENCES RARELY COME LUBRICATED💅🏽