If you both managed to go beyond the silly tsundere tropes, you could see what the two of them's point of misunderstanding is.
Phos started out wanting to save Shinsha from her condition out of the compassion he feels, but then he couldn't help to frame it as "I have been saved so I'm going to do the same", taking a karmic debt over his shoulder. Even when he can't really remember their previous interactions, he values Shinsha and wants to do things FOR him. Shinsha always saw Phos as the only one who extensively cared for him to the point of suffering and his tsundere act is in fact swinging between "I don't need this because it'll only bring more suffering to me and Phos too" and seeking an actual relationship that doesn't need to concern with saving each other.
The tanoshii shigoto is a metaphor of this: Phos can't help seeing it as an actual way of redeeming Shinsha (focusing on the "job" part), while Shinsha, who has started all of his night shifts by saying "shigoto" for centuries because he can't fathom that it's okay to just exist without taking karmic debts (as Sensei asked him to), asks where did the tanoshii go because he's focused on the unconditional relationship part. Too bad Phos can't see it because he's too taken by his need to save him because he sees himself as too undeserving to just 'hang out' and be happy. Hope this helped.