The day was a haze, I barely noticed as I moved through my morning classes. I couldn't tell you what we covered, or even who was there. All I know is that my lack of attention must not have been visible because Mr. Parker would have pitched a fit and I would have remembered that. At lunch the conversation was dull and pointless. I wanted to scream at Mike and Jessica to shut up. Every word was a painful reminder of how monotonous and useless my life had become. I just wanted everyone and everything to leave me alone.
Then it happened. In biology I glanced across the room and looked at a boy, I don't know his name, at the same time he looked at me. Our eyes met, and he smiled. Then he looked away. Something changed when he smiled. The room came into focus, the colors deepened, the ... I don't know. All of a sudden things were just better. I felt something other than boredom.
I don't know if that's ever happened before. I mean, sure, reading a book, watching a movie, maybe looking at a sunset. I'm fully capable of feeling good. But to be sitting in a classroom, surrounded by people I don't know, in the middle of a boring lecture, and to suddenly feel like things aren't wrong. It's ... I can't describe it.
When he smiled at me I felt good. That word doesn't do the feeling justice. There are no words. I've never really felt so right.
I can't wait for biology class tomorrow.
Ok, I think I went a little overboard there, but the point is that if you have something like that for a book then, even if she never gets around to asking him out on a date, him disappearing could absolutely throw her into a depression. Because in this hypothetical story, the important thing isn't that there is an actual relationship, it is that narrator has found something that makes her feel good. Take that thing away and she'll be in freefall again*.
Of course one could just as easily make a story where the something isn't a human being but instead, say, the chess club. Or a tv show, or a work of fiction, or anything really.
It also should be noted that there's a giant range such a thing could fill from successful coping mechanism to unhealthy obsession.
*Or she'll set out on a mission to figure out what happened to him and save him if necessary and we've suddenly got ourselves a ... well it could be any number of things, but I was thinking an action movie I don't have all the details, or indeed any of them, but if this is inspired by Twilight then he should be a vampire.
So I'm guessing that he's either been captured or is in the process of being chased by an elite group of vampire hunters, and then 17 year old protagonist goes off to save him perhaps with some minor handwaving about how her police chief parent taught her how to fight so she would never be a victim (or she just likes karate, whatever seems like a semi-plausible explanation for her being an action hero) and then there's stuff about explosions and crossbows and whatnot until she rescues him.
It does well at the box office (second highest grossing movie on it's opening weekend) and is held up as proof that a fully clothed female character can be the lead in a commercially viable action movie.