Why did the ibis die in The Scarlet Ibis?

the scarlet ibis is sad because the brother has a brother named Doodle and throughout the story he is mean to him and pushing him harder to do things for selfish reasons. such as forcing Doodle to touch his coffin and teaching Doodle to walk because he didnt want a "crippled" brother. Doodle died at the end of the story because his brother pushed him too hard to do thiings such as rowing and Doodle died. what really broke my heart was whenever Doodle would say,"don't leave me big brother." Doodle was only 6 and his brother found him dead during a storm when he was trying to run away from him.