Why is Harry more affected by events in Harry Potter and the Order of the Phoenix than in any of the previous novels?
Being 15 is hard for anyone. I remember reading a review of this book when it first came out, complaining that Harry is whiny and depressing. Isn't everyone at this age? Once he enters his fifth year, more is expected of Harry than ever before. Up until now, he has had protectors and little adventures. Things turn dark for him after his duel with Voldemort, and Cedric's death. He is dealing with the regular teenage problems of love, friends, adults' expectations and deciding who he is. On top of that, he feels the weight of the hero on his shoulders. He has doubts. He knows he needs to save the world, and he is not sure he can.
Harry is more aware of what he is facing and what he is losing. This is the first time that Harry went for a whole summer with no contact from the wizarding world. Harry finally found a parental figure in Sirius, and that made him happy. In this book all of his happiness is being taken from him.
Harry is affected more by events in this book than in any of the previous novels because he faces more disastrous circumstances in this book than in any of the other novels, and considering the circumstances of The Goblet of Fire, that is saying a lot. To begin with, he suffers a dementor attack in the Muggle world only to be charged and threatened with prison; he is tortured over and over with nightmares sent by Voldemort. Perhaps the worst events of the year for Harry come in the form of being kicked off the Quidditch team, his one source of escape, his detention torture sessions with Umbridge, in which he is forced to cut “I will not tell lies.” into his hand over and over, and finally the ministry battle and losing the one person he has loved as a parent. Harry faces so much devastation in his fifth year; it is hard to believe that he comes back in his sixth year with as little damage as he does. He is a very resilient and strong young man, with the help of his friends.
The novel that immediately preceded Harry Potter and the Order of the Phoenixwas Harry Potter and the Goblet of Fire, in which Harry witnessed Cedric dying and Voldemort coming back to life. Since Voldemort has now gained a body for himself, the threat of his power seems all the more real, especially when punctuated by the first death of his coming being a fellow Hogwarts student who Harry had grown to befriend. This experience also alienates Harry from his fellow classmates. Who else can understand the horror of seeing someone drop dead in a second and dealing with the subsequent nightmares, not to mention constant escapades into Voldemort's mind and seeing strange (albeit gentle) creatures such as thestrals. It is not a stretch to say that the Harry in the fifth book is a much different Harry than the one in previous books. Together, all these factors may have taken a toll on him and caused him to react more emotionally to all the events.
For me, Harry Potter and the Order of the Phoenix was the book really fleshed out Harry, giving him an emotional depth that I really hadn't seen in him before. In the previous books, Harry had just been a decent boy, certainly talented but without much emotional range, who does the right thing just because it's the right thing to do. In this book however, Harry's emotions all over the place. This is certainly one of the results of the disturbing visions that Voldemort has been forcing him to see. His visions of the Department of Mysteries are not only frightening, but also intriguing. Harry is also very disturbed when he believes that he was the one who had attacked Ron's father. Such direct access to Voldemort mind, even to the point that Harry believes he is Voldemort himself, is obviously going to be a disturbing experience.
Also, it's starting from this book that the Ministry of Magic becomes an antagonist for Harry. Finding that your own government is against you is obviously going to be a disturbing experience. Harry is forced to endure not only slander from the nation's most prominent newspaper, but also ridicule and mistrust from almost the entire Hogwarts student population, including even people who he had previously thought of as friends.
The Order of the Phoenix is definitely the book where things become more real, and much darker, for Harry. It is no wonder that he reacted in the way he did to the rapidly changing world around him.
Uh... probably b/c this is like the most important book in the series ever!!! Harry finds out he is the Chosen One and flips out like any teenager would. Serius Dies! (so sad) Voldemort has shown himself to the public, him being called a liar for half the year might have stirred him up a bit too. But really it mostly that he realized that either he or Voldemort has to die. The weight of not only the wizard world but the muggle world as well. Pretty big stuff he has to deal with. I would call that MAJOR.