These two. These two right here are the core of this story. And this arc helps illustrate that point. This is Raku and Chitoge's story. I like Kosaki a lot, don't get me wrong, but she's pretty much a 'losing heroine' that hasn't quite realized it yet, and neither has Raku realized where his heart truly lies.
Kosaki Onodera represents the life he had before Chitoge. She represents the dull existence he had and longed for, before all the excitement started with the Beehives and the police and all that. But that peace is all gone. He can't reclaim those sleepy days by chasing Kosaki. If he did, the town would fall apart, and he'd also realize what he'd lose in the process was far more important than what he'd gain.
It's kinda like how, in Remake Our Life! (I watched the anime, at least), the future that Kyouka forges by meddling in the University lives of so many talented creators caused their careers to never truly take off, and though he had a happy married life with Aki, he wept for the great creations that never happened, and never would happen, in that timeline from Aki, Nanako, or Tsurayuki. So he chose to go back and try again, to encourage them to be their best without him trying to take care of everything for them. To find their greatness on their own without relying on his tireless support, that they never needed in the first place.