All of Thirteen Reasons Why is either spoken or thought. And, we don't know about you, but Shmoop sure doesn't always speak or think in a very polished way. Turns out, neither do Hannah or Clay. For this reason, the style of the novel is very choppy, with short sentences and even a few fragments. Take a look at this:
That's why you did it. You wanted your world to collapse around you. You wanted everything to get as dark as possible. And Bryce, you knew, could help you do that. (14.84)
If Shmoop were a composition teacher, we might suggest that Clay combine some of these sentences. Here, we'll give it a shot: "You wanted your world to collapse around you, and that's why you did it. You knew Bryce could help you get everything as dark as possible." Doesn't have the same ring to it, does it? Sometimes short sentences have a certain impact (ominous, perhaps?) that you can't get otherwise.
And hey, Clay is thinking, not writing; that's why the style is conversational. When you think about it, Clay and Hannah are having a conversation, even if they're not actually there talking to each other. This more casual style definitely helps us relate to them – after all, we wouldn't be as big a fan of Clay if he were a total grammar snob.