Although the book is primarily told in a first-person perspective, it's not from the a p.o.v. you generally hear from in literature: an invading alien species. Our narrator, Wanda, has taken possession of a human body, Melanie, and 95% of the thoughts we hear in the book are Wanda's.
Meyer plays with both the p.o.v. and tense at different times in the narrative. Melanie's memories are told in first-person from her point of view, but they're told in present tense. This gives her memories a feeling of immediacy. How many times has a strong memory made you feel like you were right there?
She also uses the plural "we" at certain points in the narrative, indicating that Melanie and Wanda are actually cooperating. This happens for the first time when they're about to die in the desert: "We only lasted a few hours" (12.46), Wanda says, uniting the voices in her head in death.
When they first encounter Jeb and the other survivors, she talks in "we" for a bit, until the humans make it clear that she is not exactly a guest of honor in their society. At that point, she goes back to "I," signifying that she stands alone against the humans, her enemies, despite Melanie's voice living inside her.
The point? These narrative tricks show us when Melanie and Wanda are at their most connected, and when they're at odds with one another.