Why does the narrarator teach doodle to walk, and why does he cry when his family congratulates him for his effort?
The narrator/brother teaches Doodle to walk because Doodle will soon start school, and he is afraid that Doodle will embarrass him terribly there. On the day that Doodle demonstrates his accomplishment, the brother cries in shame when complimented because his motivation was entirely selfish.
When Doodle was five years old, I was embarrassed at having a brother of that age who couldn't walk, so I set out to teach him.
Day after day, the brother takes Doodle to the palmetto thicket near Old Woman Swamp and stands Doodle on his feet, commanding him to walk. When Doodle whines, the brother makes him feel ashamed by describing the two of them as old men and he trying to still pull Doodle in the cart. Then Doodle feels guilty and promises to make more attempts.
When Doodle finally succeeds at taking a few steps, the brothers decide to wait until Doodle is more accomplished before informing their parents.
On Doodle's sixth birthday, the brothers decide to reveal the secret accomplishment of Doodle. When Doodle walks to his place at the breakfast table, the mother starts to cry, Aunt Lacy gives praise to God, and the father hugs the brother. Then, the brother cries because he is ashamed to accept their compliments when he knows that it was his selfish pride, not brotherly love, which impelled him to teach Doodle.
Significantly, the brother later remarks,
I did not know then that pride is a wonderful, terrible thing, a seed that bears two vines, life and death.
Indeed, it is this pride that has driven the brother to teach Doodle to walk as well as to cause Doodle's death in the end.