I think the rock told this to kihoon not yeon. So I think yeon didn't know at all about it.
I guess he didnt want him to suffer bc of memories. He felt guilty as he was the one who tied the thread causing the misfortune, so after learning about that and what life Kihoon had in the past I guess he decided that he was the root cause of of it, and wanted Kihoon to live his life without him tying him down or smh. We also can see Yeon thinking he isnt good enough for him either in previous or current life.
And even though Kihoon never thought about Yeon as dead weight or not good enough, he could never argue with him bc for him Yeon is almost saint like yk, so he decide to abondon his memories as per request.
Well thats what i think happened could be wrong bc i dont remwber all chapters and details but yeah :p
I'm confused. Didn't the rock give an option that misfortune will be gone and they remember each other ? Can't they just remember everything and be together, why yeon wanted kihoon to forget?