If Spirit allowed for souls to reincarnate based on their lives. Then it made sense why there is a limit. Hiro surmised the limit must have been the soul's ability to endure hardship. It is impossible for anything to remain unbreakable.
Yet Rilu went twelves times that. Just how heartless is Ethan Dragonblood to force such a life on his heir? How could he force Rilu's soul to keep reviving with no help whatsoever?
Hiro then thought about his wives, how many lives have they already endured? They already suffered so much in this life. Burned, mutilated, cursed, abandoned, Isolated. How many times have they already repeated such a cycle? The hand of fate seems to not only go against him but also against his loved ones.
