From Hoshikuzuu Scans:"You broke our promise, so I'm breaking up with you". Two people so in love in high school who decided to elope. Fifteen years later, Kusaka suddenly tells Mikuriya that they should break up. But why?! Mikuriya's so devoted to him, loves him so passionately and has never cheated on him. So he's stressing over this promise that he can't remember. It is then that he recalls a secret that he's never told Kusaka.
Nichijou Climax