This is a great exercise in analyzing the character of the narrator who struggles with conflicting feelings toward his younger brother, Sonny. Here are a couple of quotes to get you started.
When Sonny and the narrator are arguing about Sonny's desire to be a jazz musician, the narrator makes a comment to his brother:
You know people can't always do exactly what they want to do—
The narrator believes in a world of predictability and responsibility. He has chosen to be a teacher after serving in the army. This comment reflects his belief that having a predictable, dependable life is more important than following one's passions. He may not have wanted to be a teacher, but it is a responsible career choice. It indicates that he has sacrificed his own passions along the way in order to make a living, and he values that more than he values following dreams, which seem impractical.
Another comment is made in the narration, not through dialogue. In this example, the narrator is reflecting upon the time he reached out to Sonny after his own baby daughter's death:
I think I may have written Sonny the very day that little Grace was buried ... My trouble made his real.
This comment reveals the nature of pain and how it transforms one's life. When little Grace dies, the narrator is broken, and he suddenly realizes the brokenness of Sonny's entire life. This shared sense of desperation and agony draws him closer to his brother. He realizes that pain has caused much of Sonny's trouble in life, and he also wants his brother with him to share the burden of his own deep pain. This is the nature of life, and he realizes that he has robbed his brother of the ability to share life's pain by distancing himself from Sonny.