For necomers to the series, Uub is the reincarnation of Kid Buu, trained by Goku after the World Martial Arts Tournament. Uub realized he was weak, seeming how Goku never turned Super Saiyan when they fought and then he learned from Vegeta that he is the reincarnation of the toughest enemy they ever fought. So Uub began to harness the power of his past self, but also his personallity, resulting in his second form.