The theme of to much pride can hurt you in the long-run is introduced when the author writes "he was born when I was 6 and was, from the outset and outright disappointment.". It shows how the author was disappointed to have Doodle as a brother, and he thought that he deserved better than Doodle and his pride got in the way.
Then when he was embarrassed of having a disabled brother he decided to teach doodle to walk. the author then wrote "occasionally I become too discouraged because it didn't seem as if he was trying, and i would say, ' doodle don't you want to learn how to walk'." this shows how his pride was being lowered in his brother and he was getting discouraged because he couldn't walk and his teachings weren't working.
Towards the end he got too prideful and believed he could teach Doodle to do a plethora of thing. the author says "I should have already admitted defeat, but my pride wouldn't let me." this was towards the end when his pride was falling and it eventually lead to him running away from Doodle and doodle dying.