What motivates Buck's aggressive behavior towards Spitz?

Buck's aggressive behavior towards Spitz is primarily driven by rivalry for leadership within the sled team. As Buck becomes increasingly confident and aware of his own potential as a leader, he recognizes Spitz as the current alpha. Their rivalry is intensified by the harsh environment and the competition for dominance among the pack, leading Buck to challenge Spitz in order to assert his own position. This struggle reflects the natural hierarchy and social dynamics of animal behavior, where individuals compete for leadership roles to gain better access to resources and ensure their survival. Spitz's role as a dominant figure signifies a barrier to Buck's aspirations, prompting him to engage in aggressive acts as he seeks to overcome that obstacle. This competition for leadership is a critical theme in the story, showcasing Buck's transformation and adaptability in the wild.