My father in law, rest his soul, was not always an easy man to be around, but the holidays were particularly tough for him due to childhood trauma.
He was not, by any means, an introspective person, and I doubt it would have even occurred to him to try to get to the root of his seasonal sadness and grumpiness.
Even if it had, he would have dismissed it as psychobabble.
Some people are just stuck, for whatever reason, and can’t move forward, more’s the pity.
It was always nice for us to travel, as we did frequently, during the holidays, so we didn’t have to deal with him until after , when he was more reasonable about things.