The paragraphs in question not only create contrasts between themselves, they create contrasts between Alexie and his peers. The paragraphs show that different choices lead to different consequences.
In paragraph 29, Alexie talks about playing basketball. In the paragraph before, he had discussed not being good at it, which is why the quote begins with a "but":
But it felt good, that ball in my hands, all those possibilities and angles. It was mathematics, geometry. It was beautiful.
In paragraph 29, we see Alexie engage physically with the ball and find beauty in it. In paragraph 31, the same structure is used, but this time, the narrative voice states the following about basketball:
But it felt good, that buzz in his head, all those colors and noises. It was chemistry, biology. It was beautiful.
Here, it is not only the physicality of the ball that feels good, but also, mentally, it is the "buzz in his head," that appeals to him. The two paragraphs emphasize that he makes the choice to engage both physically and mentally with the sport: to be present to it in all ways.
In between, in paragraph 30, his cousin, Steven Ford, sniffs glue ("rubber cement") and escapes from his problems. His ears ring and everyone feels "far away. " This is far different from the "buzz" Alexie experiences from basketball. Alexie had the option to escape, but chooses instead to stay present with the world.
Likewise, Alexie is his class valedictorian as he graduates from a new school, his "farm town" high school. He writes in paragraph 70:
I try to remain stoic for the photographers as I look toward the future.
Paragraph 72 shows the contrasting reaction of his new set of classmates, many of whom are graduating inadequately educated or illiterate:
They smile for the photographer as they look back toward tradition.
Alexie's future is full of possibilities—possibilities he has because he decided to engage with the world—and is highlighted against the limited possibilities his classmates have. The sentence structure in paragraphs 70 and 72 are the same but the content is the opposite. In both the reservation school and the town school Alexie differs from his peers in attitude and orientation. The effect is to help the reader appreciate the choices Alexie made.