Incidentally, I’ve been trying to think for a while what set FW off in January 1739.

I noticed that he was set off in January 1736 and January 1739. This coincides with Fritz visiting Berlin. cahn and I had a conversation (related to her Yuletide Madness fic) recently about how dysfunctional families set up all these expectations about how GREAT it's going to be to get together and see each other again, and the disappointment sets in early and increases throughout the visit. Wusterhausen is only a particularly egregious example.

Going from warm and fuzzies in December, in the first part of the visit, to disillusionment in January, is straight out of the dysfunctional family playbook, to borrow a phrase from rachelmanija.