Answer: Because Doodle is handicapped, Brother pushes Doodle to learn how to walk. Eventually, Doodle is able to do so, and Brother pushes Doodle to achieve more.
But Brother also has a very positive impact on Doodle. Even though his brother is handicapped, he pushes him to achieve, he demands that Doodle walk. Brother pushes Doodle to do more and more strenuous activities until one day, he breaks into a run, leaving Doodle trailing.