Because of his pride, he does these things more with his own benefit in mind than his brother's.
This story is a clear condemnation of blinding and debilitating pride, since the narrator's pride brings about the eventual death of Doodle.
It can often be tempting to push ourselves and the people we love past their limits in the hopes of achieving a goal, just like what happened with Doodle and the narrator.
Sometimes this produces great results; after all, Doodle did learn to walk after working extremely hard.
This story illustrates the importance of family bonds, particularly those between brothers.
Doodle clearly looks up to the narrator, but many times over the course of the story the narrator fails to be the caring and compassionate brother he should be; instead, he is more concerned with the implications of having a disabled sibling.
But it is important to be able to recognize when too much is just too much.
The narrator was not able to see this, and he continued to push Doodle to his breaking point.
Throughout this story, the narrator allows his pride to cloud his compassion and blind him to Doodle's limitations.
He is too proud to accept having a disabled brother, and this is why he takes every measure he can to teach Doodle to do able-bodied things.