What does it mean that they all lived with lifelong regrets?

Eighteen years after his transmigration, Lin Wuhua was admitted to the National Extraordinary University, only to be falsely accused of being a "reincarnated infiltrator" and imprisoned in a heavily guarded medical facility, his life hanging by a thread. However, he unexpectedly discovers the "Book of Reincarnation," which allows him to experience different lives, complete tasks, and help the native world integrate external energy, earning rewards from the world's subconscious. Through his journey, Lin becomes a skilled warrior, mastering the "Heart Sword Technique" and transforming the Ice Sword Master into the First Sword Immortal. He also inherits the last lineage of the Demon Blade, utilizing his unique abilities to infuse souls into puppets, granting lifeless beings human emotions. As he navigates the complexities of his new world, Lin must confront the consequences of his past actions and the weight of his lifelong regrets.