Throughout the entire novel, Boxer is presented as the animal most committed to the cause of Animal Farm, and he expresses this through selfless labour on the Farm's behalf, never thinking of his own needs or wants. In this aspect, he acts as a foil to the selfish Molly, who is tempted away easily by a human and the prospect of apples and ribbons in her hair. Because he is so sacrificial, therefore, he wants to give himself to be a part of creating something lasting and a monument to Animal Farm and what the animals have achieved, in spite of the pain he is in. In the first paragraph of Chapter 9 we are told:
Boxer refused to take even a day off work, and made it a point of honour not to let it be seen that he was in pain.... He had, he said, only one real ambition left - to see the windmill well under way before he reached the age for retirement.
It is this aim, therefore, of completing the windmill, which would act as a symbol of the achievement and power of the animals, that is giving Boxer his inspiration to continue his work in spite of his poor health. It is also this attitude that makes his "end" even more shocking to us.