He just...while mood swings were rarely out of the question, it wasn't something he controlled. And even though Sirius felt a bit better and terms with Remus were most definitely improved, and Remus cared and wasn't going to stay in Morocco or anything, so it was going to be okay...he couldn't just...switch to being in a good mood, into just being them again. He wanted it, but that's not what it was between them.
"Oh," Sirius gave a little nod. It wasn't like he wanted Remus to go particularly, even if he had no idea what to say or do. "See you," Sirius repeated, not looking away until Remus was gone. Then he sighed, a tightness in his chest gone that had been pulling at him since he woke up. It wasn't for quite some time that Sirius realized he hadn't told Remus he was showing up for the symphony.