Question: how is uub the reincarnation of buu?
Answer: It is revealed here by Goku that Uub is actually the reincarnated Human form of Kid Buu, and that he was born from the wish made by Goku just before he defeated Buu with the Super Spirit Bomb. ... Uub's power increases to a point of being able to hurt Goku, although during the fight Goku never goes Super Saiyan.