Harry is such a perfect teenager in this book! He's whiny and self-important, snapping at everyone else, and thinking that 'no one understands meeeeeeeee!" Jeesh. So at times it was difficult to read because of the wanting to smack him upside the head, but that's really quite a testament to Rowling's knowledge of her own characters and her understanding of how people (especially adolescents) can change. 
 Another reason this was a difficult read was that I don't like change. And there's change all over this book. Yes, it starts on Pivet Drive and all that, but Hagrid is missing from a huge portion of the book, and then he's secretive with Harry, Ron and 'Hermy' for the rest. Dumbledore is ousted as headmaster for a while, Harry gets kissed by Cho, and Cho is all crying all over the place. It's awkward and difficult and horrifying. Just like a teenager's life should be.