This is a very important question to consider when analysing this excellent short story. Essentially, this story concerns one brother who wants to change his younger brother for his own selfish reasons. It is clear from Doodle's birth that he is not like other children, and this is a fact that the narrator, known as "Brother" in the text, finds very difficult to accept. From the beginning the narrator makes clear his hopes for a younger brother:
I thought myself pretty smart at many things,like holding my breath, running, jumping, or climbing the vines in Old Woman Swamp, and I wanted more than anything else someone to race to Horsehead Landing, someone to box with, and someone to perch with in the top fork of the great pine behind the barn, where across the fields and swamps you could see the sea.
However, clearly Doodle is not able to do this, but this does not stop the narrator from being rather cruel to him and also trying to change him into the brother that he always wanted. Note what his motivation was for teaching him to walk:
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.
It is embarrassment that leads the narrator to work so hard with his brother, and the narrator himself expresses shame when others congratulate him at his success:
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.
It is this same "pride" and "embarrassment" that leads the narrator to abandon his brother at the end of the story, angry and frustrated at his failure to train Doodle, thus leaving him to perish alone.
Thus when we think about what is the primary motivation of the narrator in "The Scarlet Ibis," we see that it is his own pride and embarrassment at having a brother like Doodle that leads him to try and change him into the brother that he always wanted--which tragically ends in Doodle's death.