Why was it such a disappointment to the older boy when the baby was born strange looking and sickly?
The older brother (the narrator, simply known as "Brother") is proud of his ability to run, jump, hold his breath, and climb. He had wanted a brother who could do all these things. He had wished for a brother who could challenge him, someone to race with and even box with. This is his definition of a brother. He even concludes this list of qualities with the statement, "I wanted a brother."
When Doodle is born "all head, with a tiny body," Brother is disappointed because he doesn't look like he'll be able to do all of these physical activities with Brother. Brother wants a playmate. But Brother also has a selfish wish and that is for Doodle to be "normal." He is embarrassed in having a younger brother who is physically disabled. His mother asks him to take Doodle everywhere and this is a burden to him. When Brother helps Doodle to learn to walk, he does it out of this selfish desire. His motivation is not to help Doodle. This is the only way that Brother can deal with his disappointment. Either he can change Doodle into his idea of a more "normal" brother or he will remain disappointed. Brother recognizes that he is doing this for selfish reasons, but he continues anyway. When he shows his family that Doodle has learned to walk, he is ashamed:
They did not know that I did it for myself, that pride, whose slave I was, spoke to me louder than all their voices, and that Doodle walked only because I was ashamed of having a crippled brother.