Pretty late reply, but I think it's probably to do with how in this story Kaguya, being spoilt and selfish, assumed that everybody was as well. Because of this, she thought that Eirin had ulterior motives, and she attempted to expose these by physically and verbally abusing her. But Eirin of course has no wish but to stay with Kaguya; and this is what is referred to at the end, since she has been able to do that for a thousand years already.