Trust me, he says.
Harry couldn't think of anyone he knew that he could trust less in this school. In all the time that Harry had known him, Malfoy had hated him. Had scorned, challenged, insulted, and just plain aggrivated him. If one removed all of the challenges which Harry had faced over the years, everything that had to do with Voltemort and his parents death, there would still remain all of the problems Malfoy had caused for him. And that alone could drive any student crazy.
So why the hell had he agreed?
Harry had found that he couldn't sleep because of that question. It haunted him. He needed a reason for his actions, and hadn't been able to find a satisfying one for his agreement. It wasn't simply because Malfoy had asked. Baring his mind to the other wasn't something that he would just allow to happen because Malfoy asked.
It wasn't because of Malfoy's ending taunt. He was so used to those that it was more of a habit than anything else to respond.
So why? Malfoy hadn't needed to beg. Hell, he hadn't even said please. The conversation had gone on as though he were asking to borrow Harry's homework, instead of his mind.
He didn't even know exactly what it would entail. Malfoy had lent him the book, to read the spell over, but it was slightly vague. From what he could gather, he would hear Malfoy's thoughts like a whisper in his ear, but only those which were directed toward him. This, and the various situations where the spell came in handy were the only real information the book gave beyond how to cast the spell in the first place.
He was a little worried about any possible side effects. Although none had been listed, that didn't mean there weren't any. What if the reversal command didn't work? Would he have Malfoy in his head forever if that were the case?
WHY had he agreed to trust Malfoy?