This story is all Hazel's. It makes perfect sense that the story would be told from her first person perspective because she has such an internalized life. She really is in her own little worlds.
We really like Hazel, so we're pretty stoked to have her as our narrator. And she's always infusing her own thoughts and personality into the story. Remember when she describes how pissed she is that people are putting inspirational messages on Gus's wall online after he dies?
That particularly galled me, because it implied the immortality of those left behind: You will live forever in my memory, because I will live forever! I AM YOUR GOD NOW, DEAD BOY! I OWN YOU! Thinking you won't die is yet another side effect of dying. (21.8)
This girl doesn't hold back.
Because she has physical constraints, Hazel relies on her words, thoughts, and feelings to give her the full experience of life. She and Augustus are often overblown and wordy, but it's how they're able to communicate their affection for each other without physical contact.
Hazel brings us into her world with her words—and it's one that's hard to leave.