Why is boxer upset after the battle?

Boxer is upset after the battle because he feels disillusioned and betrayed by the leadership of the farm. Despite his hard work and loyalty, he realizes that the promises of a better life and the ideals of Animalism have not been fulfilled. His injuries and the lack of recognition for his sacrifices contribute to his growing sense of despair and hopelessness. Ultimately, he understands that the struggle for equality and justice has been undermined by those in power.