From what I understand Sajou just suddenly realizes that he's becoming an adult now, and it really caught up to him how he has no idea what he wants to do in the future minus marrying Kusakabe. I feel that the main plot this story has is about becoming an independent adult and all that- from that we can see that all the thoughts about their relationship comes from flashbacks- their relationship was built on the foundation of their youth when they were high schoolers and now they aren't, and the idea really catches up to Sajou when he was at that gathering.
And Kusakabe overreacted with just going about the breakup.
It didn’t say explicitly in the manga, but my take from there is that they are simply in the cooling period. They have been together for about 4 years at this point. The honeymoon period has long passed, the long distance, university work, job hunting stress, future plan... everything must have added up and Sajou just... quit. Or feels like quitting. It’s actually very common for long time couples, and Sajou and Kusakabe don’t even have the luxury of spending time together frequently enough for a serious talk. To be very honest, Kusakabe giving up professional musical career and juggling between part time jobs doesn’t provide a bright future prospective either. As much as it breaks my heart, I’m thankful for this development because its very realistic and relatable.
I agree with you. If they can survive this challenge, their relationship will come out stronger than ever. The bond is there, and I don’t want to spoil you, but just trust Nakamura-sensei to know what is the best for our boys.
Had to reread Ch.1 cuz I couldn't remember why they broke up. Now I know why. There isn't really a proper reason. You sit there reading it and think "what the hell just happened"!? xD guuuuys, why're you creating such a drama -.-