At first I thought that volume 5 was a pretty random arc to follow Yuzu and Mei first getting together, but now it feelings like it's been building to the current development all the time.
Volume 5 - Mitsuko forces Yuzu and Mei to make hard choices, but also introduces the concept of creating your own path with a strong will. Volume also showed how much Mei loved the school, even having to hide her grandpa's photo when kissing Yuzu in his office.
Volume 6 - Yuzu and Mei's relationship intensifies and the ring is given to Mei, which triggers her deeper feelings. Mei acting awkward why comforting Yuzu in the library now has a more negative meaning in hindsight.
Volume 7 - Mei starts actively trying to figure out if she loves Yuzu. By the end of the volume she realizes that she does.
Volume 8 - Sexual tension reaches its peak and is really the calm before the storm of volume 9.
Volume 9 - Everything starts to go to hell when Matsuri learns of Mei's engagement. Their friends worry and ponder their future, Yuzu is oblivious to why Mei is acting weird, while Mei is obviously torn. Eventually the guilt overwhelms Mei when Yuzu is thinking of their future together, so timeskip to after Mei bails in order to fulfill her duties. We get a short where are they now of the rest of the cast.