...it makes sense in my head, but I find myself incapable of expressing how, gah! Perhaps Sam's attempts to go 'look, Dean, it's all right, you don't have to beat yourself up about this, it wasn't you' frustrate Dean because he feels that he's making it more difficult for Sam to get over what happened; if Sam has to put on an 'everything's fine' act to help Dean, it means that he doesn't have the time to come to terms with it himself.