Why does Doodle cooperate when his brother forces him to learn to walk?

In the “Scarlet Ibis” by James Hurst, Doodle does learn to walk, but why? Doodle cooperates with Brother’s efforts to teach him to walk after Brother makes him feel guilty. When Doodle tires of trying to stand or questions his need to walk, Brother becomes insistent. Brother says,

Then I'd paint for him a picture of us as old men, white-haired, him with a long white beard and me still pulling him around in the go-cart. This never failed to make him try again.

Doodle does not like the picture that Brother paints of their future. With Brother’s persistent efforts, Doodle learns to stand which makes both boys ecstatic. Doodle gets caught up in Brother’s enthusiasm and finds a reason to continue with the hard work of learning to walk. The boys decide to keep walking practice a secret until they are successful. Again, these clandestine activities keep Doodle interested in Brother’s efforts. Failure is not an option at this point.