Straight from the Teenager's Mouth
Reading Story of a Girl feels a lot like talking to your best friend, especially if your best friend is totally angst-ridden and conflicted. Like most teens, Deanna uses plenty of slang. Just look at how she breaks it down for Lee:
"You want my advice? My advice is that you're not missing anything and in a couple of years you'll go to college and me and Jason will be here in Pathetica working crap-ass jobs and hanging out at Denny's, so why waste your time? On either of us." (5.105)
Calling your hometown "Pathetica" and your job "crap-ass" is about as teenager-y as it gets. Right? Right.
Plus we actually get to see into Deanna's journal, where she fictionalizes her memories through the story of an unnamed girl: "The girl remembered running down the driveway toward him, cement cold under her tiny feet" (2.8). She's giving us a very intimate look into her thoughts about her relationship with her dad, which is much more immediate in Deanna's own words. We feel for her in a way we probably wouldn't if it were written in a more formal, more adult voice.