After finally destroying Kid Buu at the end of a long, protracted battle, Goku wished that he would reincarnate as a good person in his next life. That wish was granted, and at the end of Dragon Ball Z Goku met Uub, the human reincarnation of Buu. Realizing that he had naturally become a strong fighter, Goku decided to take Uub and train him up into someone who could step in as Earth's guardian once he was gone.