Why does the narrator teach doodle to walk and why does he cry when his family congratulated him for his efforts?

The narrator, who is also Doodle's brother, teaches Doodle to walk because Doodle will soon start school and the narrator fears Doodle will embarrass him there. When Doodle shows his family he can walk, the narrator cries because he feels ashamed that his family is complimenting him because the only reason he taught Doodle to walk was for selfish reasons.