Above all, this sentence shows how Boxer has fully internalized the values of Animalism. He is motivated to work ever harder based on a striking commitment to the society, and to his belief that he can improve the lives of others simply through hard work. Indeed, he is quite effective in this endeavor, and is able to aid the animals in producing an excellent harvest.
Yet the phrase also speaks to his narrow-minded perspective: Boxer does not consider other ways that he could approach life, but rather identifies fully with a single quality: his physical strength. Orwell renders him a caricature of how adherents to socialism were required to behave dependently: the best workers were instructed not to reflect on their position in the system, but simply drive themselves to work ever harder. Boxer thus demonstrates both the efficacy of a revolution like that in the USSR, as well as the significant drawbacks to such a structure.