I am on chapter 9. I don't have a problem with the pacing of the relationship (because I have seen other mangas with that sort of pacing), so much as I don't feel emotionally attached to either of the characters or their relationship. Maybe if they had shown more moments from their past as childhood friends, or more emotions in general between the two main characters, I would find myself more attached.
I like to imagine that in their next lives Hajin is born as the older brother of Sihyeon and Ahyeon and protects the two of them while Sihyeon and Ahyeon adore Hajin and give him the childhood full of warmth and love that Hajin was deprived of.
Why must you make me sob? I truly hope for all of them to have great loving parents also... just a warm home...