Responses
They didn't have to lose their memory, the red thread rock gave them a choice to keep or lose their memories. Yeon chose to keep his and Kihoon chose to lose his because that's what Yeon wanted. Both of them very well could have kept their memory if they wanted
I still don't really understand why the red rock made them lose their memory? Like didn't it say you could keep your memory or was yeon just trying to make up for the misfortune of kihoon? I guess now that I'm writing it that explains why, but still curious what others think