It hurts when he says "won't you forget about me?" Because despite all we've seen Shun still feels that he can be forgotten by Chokki.
And what does he get in response? "You sure love the hyper go home rules little guy XD"! Not her reassuring Shun to some way, in some degree that he is unforgettable outside of their "club" and him being a "junior". There was no "I will never forget you as an individual person" but a sort of "you'll be part of a fond amalgam of feelings I had in school". The "bond" comment was predicated on the club, and followed by him being a junior. And maybe she was being deep or coy, but unfortunately we know Shun's thoughts: "precious junior huh?" That sounds pretty deflated to me.
Listen, I'd be mad at my bro if I said, "Hey man, I hope you don't forget about me" and I get a fist bump with a "You're a valued member of this organization and our shared commitment to the task enshrined by our core competencies".
I'm starting to feel bad for Shun. Chokki is more complicated than your average person. They have just rolled with things in their care free days, but have we seen them really overcome a struggle? What if there is some inevitable stress that a true relationship brings where you need someone to rely on or be accountable. A previous poster joked about her letting Shun know she's in labor by carrier pigeon, but imagine them handling any sort of mundane family crisis. It's comedy from the outside but tragedy on the inside if they have a future together.