I don't think anyone is asserting that he doesn't have the mental capacity to do it. However, think back to comments that his agent (or he) made when it was announced he'd be starting in LF. He wasn't too pleased to be moved away from the glory position. All the glory he enjoyed in the first season has been chipped away at in different ways. My thinking is simply that he's not being treated as the second coming of Mantle and every great in the HOF with on-the-field organizational moves the Angels have made - and that's a dampener on his excitement, enthusiasm and drive.
It could be argued that that's completely out of this world, but when we have as limited information as we do about his psyche and personality, who's to say its anymore wrong than it could be right?