Souko Morinaga was the reason as to why the infinitely powerful, dangerous forbidden Book of Friends was created by its young adult creator of a topmost s exorcist. This is because her meetings with Reiko Natsume gave the latter the idea of challenging youkai to various games, duels and "fights" in exchange for their name as a way to kill time she never would have with fellow humans.

(Source: Natsume Yuujinchou Wiki)