Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
First Person (Central) / Lennie
Imagine if this story wasn't in Lennie's voice, if it had a distant narrator that only described her actions—as in her dating-two-boys-at-once-and-lying-about-it actions.
Admit it: You'd think she was a terrible person. Or, if you're sympathetic, maybe you'd think, "That girl has some emotional problems." This is probably why Nelson takes readers deep inside Lennie's head, following her train of thoughts and memories. Readers know exactly what Lennie's thinking when she avoids Sarah or kisses Toby, and it makes a big difference in the opinion we form of her.
When Lennie tries to explain to Joe why she kissed Toby, she thinks, "My sister was pregnant, I'm about to say in explanation, but how would that explain anything? I'm desperate for him to get it, but I don't get it" (24.40). With Lennie narrating, her confusion becomes our confusion, so we're all in it together, trying to solve the mystery of why Lennie is acting the way she does at any point, and rooting for her to find her footing.