In "The Scarlet Ibis," what does the narrator's pride have to do with the plot?

Brother has a younger brother.  His name is Doodle, and Doodle is mentally and physically handicapped.  The doctors never expected him to live long, but Doodle defied that prediction.  Other predictions followed.  He would never be able to crawl.  If he did manage to crawl, the strain on his heart would be too great, and Doodle would die.  That didn't happen either.  
By the time that Doodle was five, Brother was embarrassed that he didn't have a brother that could walk.  His pride made him ashamed of Doodle.  Brother absolutely could have distanced himself from Doodle at this moment, but he does the complete opposite.  Brother works continuously with Doodle for weeks in order to teach him how to stand up and walk.  The family is amazed at what Brother was able to accomplish, and the family praised him for his love of Doodle.  But Brother tells his readers that it wasn't love that motivated him.  It was pride.  

They did not know that I did it for myself, that pride, whose slave I was, spoke to me louder than all their voices, and that Doodle walked only because I was ashamed of having a crippled brother.