Have you never seen or read anything good in your life?
What I mean is that characters have distinct ways of speaking tied into their background and personality. Instead of that being the source of the authentic feeling in the dialogue, Czajkowsk uses her own contemporary slang and speech behavior.
As for setting: there's almost no real sense of what the characters lives were like before the plot. It'd be easily to flesh the setting out in two or three scenes more early on, and a few scenes along the way. You don't need encyclopedia articles.
As for the characterization, you have awkward girl, brooding guy, vain girl, etc. They are given the illusion of depth by the occasional incorporation of contrary trait, anger, insecurity, etc. You dont have to have the characters explain their motivations. It comes out through their behaviors.