Arthur doesn't know how much of this was planned in advance, but Cass is here at the appointed resurrection time, and she clearly knew about it before Arthur did. That fact and the warmth suddenly in Crichton's tone make something unpleasant happen in Arthur's stomach. He buries himself a little more busily in the wardrobe, to hide the change in his expression.

This isn't about him. This is about Crichton being free of the voice in his head, a thing unironically normal to want and possible to achieve.

He remembers the day each of them learned about the other's passenger, and its crimes, and their hope to be permanently free. It's taken a while, but look: that's both of them now, free. And he feels a strange, sad mirror of the anger that Crichton showed when Arthur admitted to trying to keep his passenger around.

...wow that was a sudden mood swing, don't mind him he's just really invested in finding the right Hawaiian shirt to match these pyjamas.