here's my theory:
I think the guy with scare tried to commit suicide in high-school (which explain the scar on his face) after Dohyun toke advantage of him and throw him away and left him broken-heart and lonely even when the guy with scare did everything he could to satisfy him and keep him by his side since he dose not want to be lonely again (obviously Dohyun knew that and manipulate him to have s*x with him).
I guess when Dohyun knew about it he felt so much regret of the fact that he pushed him to committing suicide to the point he made himself forget so he can stop the feeling of regret and sorrow. (it really happen IRL. Its kind of reaction to help your mind get over painful things that happened)
I guess it's all starting to make sense now. But how can the "victim" not remember such strong events? All those times in the past would be pretty memorable don't you think?