Something occurred to me. Spoiler tagging the speculation:
Mokou's motive for feuding with Kaguya was the death of her father, a result of him undertaking Kaguya's impossible requests. At this point in the story he is both physically and emotionally distant, both of which could close as a result of Mokou's letters. Those letters would not have been written if Mokou and Kaguya hadn't met, which wouldn't have happened if Mokou and Nitori hadn't gone to the market that day. It seems that Nitori is the root cause of the entire immortal feud.
Although there's plenty of room for the artist to throw curveballs.
In this world, there is a thing called "destiny."It remains an unwavering pillar of rational thought.I can see it.It is a dreadfully heavy sight, that pillar.As I mentioned earlier, her choice to start playing repairman has the possibility to affect the present.No matter how often we may appear to tear down its walls,I know what you wish to say.Such a thought causes me great anxiety.A massive monolith towering between those two girls.
