The author, with this metaphor, is trying to convey that the goal of reconciliation is an illusion, that it doesn't work in practical terms and that it doesn't change anything. When he says that it is an "agreeable dream", he is trying to express that although the reconciliation and its necessity is something agreed on, it is nothing but a dream, something that isn't real or that can't be reached in reality, in practical terms. It is just a wish, a projection, or an illusion. Also, not only is it a dream but also it is a dead dream or something like a memory of a dream. That creates a feeling of distance even bigger from reality. The goal of reconciliation is wanted, but also known to be impossible to reach. Therefore, in the end, one ends up being "left" by this illusion where they once were, and nothing is really changed. The reconciliation doesn't happen, because things would still be the same, implying that any guilt or argument that once existed would still exist after facing the illusion of reconciliation.