Why does the narrator pull doodle in a cart in 'The Scarlet Ibis'?

Doodle's go-cart symbolizes the burden he places on his older brother. Brother wanted a younger brother with whom he could run, swim and play with. Instead Doodle was born handicapped. Their mother insisted that Brother take Doodle along with him whenever he went out to play; however, Brother could do so only by pulling Doodle around in the cart. In other Words, Doodle is holding Brother back from doing the things he wants to do like a millstone around his neck