I feel the exact same way. I think it's because we really haven't seen them do anything besides have sex. I mean they haven't seen each other since middle school and their first date was just Netflix and chill so It's like they hardly know much about each other.
I think the story is more about them learning how to be honest and to say what they're thinking. It seems like the point of every chapter is the importance of communication and we learn more about their feelings every time they do. I feel like that may be why we aren't exactly sure yet as far as the "why". We don't actually get to know the deep down reasons for it until they actually say it to one another, I think thats the authors point. For example, Nishi keeps saying he hasn't told Komiya the other reasons besides the nape thing.
Woah, I never noticed the rib cage til I read the comments
Also, is it just me or is something missing? I'm not really feeling the love/connection between the two, and with the way they are now, it just feels like they won't last long. Orr it'll just be a super bumpy relationship throughout