Somehow, that doesn't actually surprise him as much as the news of the training itself. He could see certain qualities Stiles had that would make it come easier.
But when he asks if it would have mattered, it's all he can do to not punch him, and he barely manages not to because the question is maddening. "Of course it would have mattered! I would have known why you were gone. I would have known you were safe. That it was for a reason, and not just because you couldn't handle being around us any more." The more he goes on, the quieter his voice seems to get until it's a steady few octaves above a whisper. "I wouldn't have wasted weeks watching that tape over and over again to find clues...or all the time I spent trying to piece together some kind of-- reason or pattern to the postcards you sent..." He might have stopped blaming himself, thinking somehow it was his fault that Stiles wasn't coming back.
Tears sting his eyes, but he doesn't let them fall. "You don't understand what I went through...but--" he shakes his head and gets to his feet. "I guess none of it matters now. I guess it never really did." But Scott can't stand to look at him anymore, to stand there and listen to excuses. He just casts Stiles one last lingering look, eyes bright with tears and shakes his head as he turns and walks away.