Throughout "Sonny's Blues," the external conflict between the narrator and Sonny mostly relates to Sonny's refusal to attend school in favor of spending time with his music. This conflict is the result of the difference in personality between the two siblings. The narrator is a more conventional person with a steady, respectable job and a family. Sonny is more artistic and restless, which makes him a talented musician but also puts him at odds with his brother, who wants to keep him out of trouble.

Sonny skipped school because he was more interested in his art. He admitted to going to Greenwich Village to spend time with other musicians. He practiced his skills with the piano endlessly. This behavior frustrated the narrator because he promised his mother he would do his best to watch out for Sonny. In the past, his mother, seeing similarities between Sonny and her doomed brother-in-law, was afraid that Sonny's passionate nature would eventually kill him. The narrator shares that concern.

As a result of his concerns, the narrator sees Sonny's actions as self-sabotage, but Sonny sees his art as an escape from a painful reality. This difference in worldview almost severs their relationship, but the story ends with the narrator understanding Sonny's attachment to his music in a far more sympathetic way.