MC is old enough that he's still not going to be seeing her as a love interest anytime soon despite her efforts, if ever at all. Especially since he still thinks of himself with that age's mentality. I think the fan service part was more meant as a comedy bit rather than true fan service b/c of that. We all know he doesn't see her in that light (he watched her grandparents grow up as kids after all), so seeing the failure and dejected look as he just brushes it off was kinda funny.