Brother's idea of an ideal brother is someone who is a mental and physical competitor. He wants someone who can keep up with him, challenge him, someone who can do all the things that he can do. Early in the story, Brother notes that Doodle is a disappointment, primarily because he can not do any of the things Brother wants to do. He even claims to have had thoughts of smothering Doodle with a pillow; that is, until he realizes that Doodle is "all there." Brother says that Doodle became one of the family when he began to crawl. But even after this, from Brother's perspective, Doodle is an outcast. Doodle is a burden to Brother.
Doodle simply wants a companion. When Brother scares Doodle with his coffin, Doodle cries out for Brother never to leave him. Doodle's ideal brother is someone who will not scare him, someone who will protect him and never leave him. Brother is a decent companion outwardly but Doodle doesn't fully realize that Brother is only helping him to make Doodle more "normal."
At first it seems admirable that Brother begins teaching Doodle to walk. But after their first display of progress, Brother reveals that he is only teaching him because he is ashamed of having a crippled brother. Brother is also directed by his own pride. Just as he wants Doodle to be "normal," he doesn't want to fail in making that happen because of his own pride.
We have to remember that Brother is a child himself, but that doesn't really excuse his selfish motivations. However, he does cry when he realizes his motivation for helping Doodle is selfish. And he does cry when he finds Doodle in the end.