It is a Climax of a story
Tze-Yo-Tzuh allows the Monkey to fly all the way to the five pillars of gold. The Monkey went "past the edges of the universe" and "through the boundaries of reality itself". When the Monkey reached the five pillars of gold, he labeled one of the pillars with "The Great Sage Equal of Heaven was here". He then pees on the pillar.
When the Monkey went back, Tze-Yo-Tzuh showed him the pee and the tag on his left hand, proving he has " all of existence forever within the reach of his hand.
Because, Buck wanted to Lead the sled himself. He thought that since he beat and killed Spitz, then he had earned that right only.
