When Buu first appears out of his Ball that he has been sealed inside for a million years (according to Babidi), he acts very childish due to his absorbing of the Grand Supreme Kai: Daikaioh.
Ironically, in his original state (called Kid Buu) he is more powerful than any other of his fused states, due to his natural, deep-seeded need for destruction and without Daikaioh's good influence, he becomes and unstoppable killing machine, blowing up planet after planet, resulting in him being blown to smithereens and reforming.
Buu: WHEEEEEEEEE!!!! (starts running around in circles)
Gohan: Ooookay, i'll just kill u now. (blows up buu's head)