He did care about their friendship that's why even when she dated Eunhyeok and he was trying to move on from her, he couldn't reject her when she was coming to him for help. He said it in his monologues that all the effort he did into trying to forget her would just crumble in whenever he is with her. Which is why he decided to cut off ties with her. Sure he could've said a more proper goodbye given that the were friends. But I think he knows he won't go the moment the sees her or if she tells him to stay friends, he would've stayed. So he left to forget he completely. And now he doesn't remember her or he only recalls fragments, I think he purposely forgot her so he wouldn't be in pain anymore-- for him to completely move on. Still it seems that such fragments of memories are his solace. I think his life now makes him feel empty. That alone tells us how much he actually loved Soo Ae and how much she affected him.
He clearly didn't care abt their friendship that much, he wanted to be with her romantically, and when he noticed he couldn't get that, he started ghosting her and cut her out without a proper word. Suae and eunhyeok barely dated (officialy) they only dated for a few chapters, but there wasn't much effort to keep his friendship with her, even tho it was clear from her side that she never saw him as more than a friend. If he properly loved her he would have been happy to see her happy even if it hurts him too. I know love is complex and can hurt but disappearing her the second she's with another guy was extremely wrong. The last time they saw each other it was clear that he just expected her to pick up on his feelings (which left him a bit bitter that she didn't) even tho he never confessed. It's clear that was never only a friendship to him and that he expected more of it.
Why do ppl want her with dowha? The moment she dated eunhyeok, he didn't care abt their friendship anymore and treated her horribly. And then he left without properly communicating with her.