The entirety of the story is told from Summer's point of view, so we only get to see things as they happen to her. Because of this, we get to watch as Summer grows up, we follow over her shoulder as she copes with Aunt May's death, and we feel her helplessness as she watches Uncle Ob, unsure of the pain he's feeling and how she can help.
We also get to see Summer's perspective on Cletus change over the course of the story. At the beginning of the book she has a snotty middle schooler mentality about the weird kid at school (she doesn't want anything to do with him), but by the end she understands more about his background and his hopes and dreams, and she starts to like and respect him as a person.
In short, by utilizing first person narration, Cynthia Rylant lets us experience Summer's growing pains first-hand.