I guess Teddy shouldn't have been all that surprised about Ryder's actions, he did say that the news of his grandmother would break him, and it did. Irrationality and pride caused this in my opinion, irrationality in the sense that Ryder thought he was doing the right thing, and then spending the next 14 years either trying to forget Teddy completely through endless drugs, music and whatever else he got up to. I would say pride plays apart with Ryder unwilling to acknowledge that his decision was wrong, as well as thinking that he was the one controlling the relationship for the good of Teddy and himself. As much as I wished the ending were different, it got the point across pretty well. They have both led emotionally unfulfilling lives after the break-up, and when considering that their relationship will likely be the best relationship either of them ever had it's easy too see how depressed they both become. Teddy even highlights that he chases the same type, and also in the sense he chases the broken as he pours his time into the E.R, so as long as he goes without Ryder he will always chasing something and I don't believe he will be able to find true happiness in a life that doesn't involve Ryder.
I think you've pretty much nailed it in terms of Teddy's issue with his brother. You could probably add in some resentment issues seeing that Oliver looks up to Ryder and not to Teddy. I know Oliver's only 13 years old, but it is a bit puzzling to see how he apparently relates heavily to Ryder's music. Maybe Book Two will open this up and show Oliver's point of view a bit more, which I hope it does. As bratty as he might be, he's not going to be rational at 13 years old, and honestly the ball is in Teddy's corner to start building an actual relationship with his own brother. It's actually pathetic that the only thing that I can see holding Teddy back is Ryder, but Teddy didn't listen to his parents telling him not to be so reliant on Ryder and I guess his parents were right, not to mention truly having his best interests, not his hearts interests.
I loved the book and along with many others, I can hardly wait for book two to be released. I hope you release book two in the same fashion as this one, one chapter a day! Thanks for your dedication!