Sonny is formed by growing up in Harlem and growing up with his family. The narrator, who is Sonny's older brother, speaks about his students, people like Sonny, who turn to using "horse" because their "heads bumped abruptly against the low ceiling of their actual possibilities." Sonny feels that he can't achieve his dreams in Harlem, and part of this feeling results from his relationship with his father. At one point, the narrator says that Sonny and his father fought because "he [the father] loved Sonny so much and was frightened for him." Sonny and his father were quite similar, and the dad feared for Sonny, as his son was growing up on the rough streets of Harlem. In addition, the father knew and understood the toll that racism could take on a black man. As a result, there was always friction between Sonny and his father.
Another factor that influenced Sonny was that his father died when Sonny was 15. The narrator's mother asked him to look after Sonny, because Sonny did not have a father. His lack of a father figure as a teenager (and his mother's subsequent death) meant that Sonny was rudderless and alone at a young age. The narrator left to join the army, and Sonny lived with Isabel, the narrator's future wife, and her family. Isabel and her family didn't really understand Sonny or his interest in music, and he eventually ran off to join the navy.
In addition, Sonny attended school in Harlem, but he said that he didn't learn anything. He tells the narrator, "I ain't learning nothing you'd want me to learn" in school, meaning that school was a place where he picked up bad habits and where the quality of education was really poor. Sonny wanted to escape Harlem, and he wanted to join the military as his ticket out. All of these factors caused Sonny to feel alienated and isolated.