In your case, obviously the cause was SAD (I used to have terrible problems with that myself back in England - it's one reason I don't want to go back there). No need to invoke biochemical mumbo-jumbo.
But yes, the brain does tend to invent stories to justify its physical reactions. That's the theoretical basis of Cognitive Behavioural Therapy. And of course there is mutual interaction so that it's hard to tell what's "body", what's "brain", and what's "mind". But surely that's another reason for not taking too seriously any half-assed theory that talks only about brain chemistry and ignores everything else.
Depression is a symptom of many types of common illnesses
That's what I said. Mostly, it's a symptom/sign, not a disease process per se, and therefore not treatable as such. Any medication should be considered palliative rather than curative.