One theory argues that this custom was conceived by shogunal officials who had learned by bitter experience with Kasuga no Tsubone what can happen when the bond between the Shogun and his wet nurse becomes too strong, and thus was designed to prevent any future incidents of wet nurses using their close relationships with the Shogun to intervene in politics.
これは春日局の権勢に懲りた幕閣が、将来の将軍と乳母のつながりが深くなり、後に政治に介入されるのを避けるために考案した風習という説がある。