>>280000985
Fantasy aside, that was the best option: to let her live out her youth, and if, even after all those years, she still wanted a serious relationship, he'd take her proposal seriously. Had he married her in high school, she would have felt repressed by all the things she couldn't experience—like parties, boyfriends, friends—and would have dumped him for the first handsome guy she found.

Even knowing Daikichi, he probably felt the boy deserved something for taking the love of his life away from him.

Now think about how Kouki felt in one of the final chapters, having to carry (Daikichi hurt his back) the old man who was fucking his childhood crush, but who was also his father figure.