MM: I wasn't implying the cause of depression was always an emotional issue ("what the hell are they"=bad or abusive families, chaotic or dysfunctional living environment, bereavement, drug/alcohol abuse, bullying ...). That's just the one I've encountered most often. I was thinking of one particular person when I wrote "emotional issues" - she was fucked-up on so many different levels it's hard to know where one thread ended and another one began. My point was, there is often a cause, or causes. "Low serotonin" is not a cause, at least not an ultimate cause. Even in those cases where someone's brain has, apparently, just thrown a random wobbly, there must be a reason. Just because we don't understand what it is doesn't mean it's not there.
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.
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.