Why does the narrator teach Doodle to walk? Why does the narrator cry when his family congratulates him for his efforts?

Brother has a high opinion of his own ability to run, jump, and climb, and wants a brother with whom he can share these activities. When it becomes clear that Doodle is capable of little more than lying on a rubber sheet and crawling backwards, Brother grows ashamed of Doodle's limitations and regularly taunts him. Though Brother loves Doodle, the love is tainted with cruelty and embarrassment. I think brother had more self-interest in mind when helping Doodle.