Well I thinka lot of it was hormones but mixed up with all the things he's been through catching up with him. Like some sort of post-traumatic effect, especially after what happened in Goblet of Fire. He's been through a terrifying ordeal and the wizarding world doesn't believe him when he tells them what happened and on top of that he's being kept out of the loop by his frienRAB and Dumbledore, that'd make me pretty paranoid and angry too.
But a great deal of it is teen angst as well, hormones making him unreasonable etc.
This is why its my favourite book, Harry's so complicated and messed up!! Maybe 'not being a goody two shoes' wasn't the best description to use as he's obviously the good guy but it develops his character a great deal and it would have been unrealistic if he hadn't experienced some sort of after effects from everything he's been through, also this is the first story where he makes the wrong decisions, it shows he's not infallible. It's a very different book from the others.