The narrator cries after Doodle demonstrates that he can walk because he is proud that he has taught his brother to be able to do so. Also, he is somewhat ashamed because he has instructed Doodle from the selfish motives of being embarrassed by Doodle's failure to walk at age five.
It [teaching Doodle to walk] seemed so hopeless from the beginning that it's a miracle I didn't give up. But all of us must have something or someone to be proud of, and Doodle had become mine.
In James Hurst's story, when Doodle is born, the narrator is mortified that his baby brother is not normal. William Armstrong is "an invalid" and must lie on a rubber sheet. But, when the baby pushes himself up and recognizes his brother and smiles, the narrator takes some interest in him. Then, after his baby brother learns to turn himself over and later to crawl, the narrator determines that Doodle must learn to do normal things. The narrator/brother becomes so embarrassed to be pulling Doodle around in a cart when he is five that he becomes determined that Doodle will walk.