NSFW alert: Mike decided to shake his head in that scene because his erection slowly begun to gain big size and him shaking head was him trying to signal to his penis “not now little buddy”. He just loved seeing his big daddy, Walter White Heisenberg at work asserting his dominance over other men. It turned Little Mike on so much, and he was completely submissive to his new daddy Walter White after he killed his old daddy Grandpa Gussy Fring.