Why do people experience depression at this time of year??

[bhikkhu]

46. Yes, its mostly a biological response to the environment

...and then we cook up narratives to rationalize it, and perhaps culturally we come up with Thanksgivings and Christmasses to counteract it, somewhat.

You can see a variety of the same mechanism in teenagers, if you happen to have any around. Its perfectly ordinary to have unbalanced and unpredictable chemical conditions in the developing brain, often showing up as "bad moods". Then, an individual suffering from them will often blame the bad mood itself on some external condition which was no issue at all when she was in a "good mood". Sometimes the bad mood will be blamed on things which weren't even present until after the bad mood (etc...), but the point is that often we experience things for no particular reason other than random biology, but then we come up with narratives for why we experience them that make us feel good.

So we are somewhat addicted to pat explanations and convenient narratives, we are averse to uncertainty in spite of understanding very little (as individuals), and then we are really not nearly as good at seeing the "why" of anything as we think we are.