The parent commenter describes seasonal Winter depression. If the problem was brain chemistry, wouldn't it be with them from birth until treatment? Who has Seasonal Type 1 Diabetes, or Seasonal Dwarfism, or Seasonal Missing-an-Eye-From-Birth? Depression generally isn't something children have from birth, it's something adults get temporarily.
A few HN submissions recently are in the style "thinking about doing the thing is not doing the thing. Planning the thing is not doing the thing. . Only doing the thing is doing the thing". Comparing a brain to a large software project with bugs hiding in it, in that vein giving the computer 11 hours of 'sleep' each night is not debugging the code; overclocking or undervolting the CPU is not debugging the code; installing the latest updates and patches is not debugging the code. 'knowing there is nothing to worry about' is not debugging the code. Only debugging the code is debugging the code. Reading a badly explained idea on an internet comment and dismissing it with a mocking "thanks I'm cured" isn't debugging the code. Saying "I've tried everything" isn't debugging the code.
A more specific example, if you are going on a rollercoaster and you are experiencing physical and mental symptoms of worry - nervous, anxious, angry at the person pushing you to ride, twitching and trying to back away, eyes looking around searching for an exit, coming up with excuses to do something else instead, nervous shaking, dread tightness in the chest, affected breathing, perspiring, gritted teeth, etc. etc. then washing over all that with "I know there is nothing to worry about so this must be a problem of brain chemistry" seems a clearly incorrect conclusion.
Such a person clearly has a worry. Quite likely one that's out of proportion (e.g. "rollercoasters kill thousands of people every day!"). Possibly one that's completely incorrect (e.g. "going more than 10mph makes people's insides fall out!"). Quite likely a less clear and less obvious one - which could be anything, e.g. they saw a documentary about a rollercoaster which behaded a child and that's their only thought about rollercoasters; they saw a show about fighter pilots pulling high-G maneouvres and passing out and think that will happen to them on a big rollercoaster; they see the rollercoaster track and support flexing and don't understand that a some flexibility doesn't mean weakness; they went to a theme park as a child and older children bullied them into riding a scary ride and they wet themselves and figuratively died of shame and buried the memory; they were pushed into learning to drive at 15 by their wicked stepfather and this is pattern matching to the same kind of experience; etc. etc.
Saying "there is nothing to worry about, rollercoasters are safe enough and you know it, so your brain chemistry must be broken" isn't debugging the problem. It isn't even explaining the problem. Why would broken brain chemistry particularly affect them at a theme park, or in Winter, and not the rest of the time? How was this broken chemistry identified and measured and quantified and that hypothesis proven?
Likewise, just because the parent poster has tried sleeping and exercising and taking Vitamin D, doesn't address that humans evolved in Africa, connected to oceans and trees and tribal living, and not commuting to a fluourescent lit beige box filled with strangers writing JavaScript while being bombarded with news items about wars and genocides and stories of how everyone else is having a wonderful Christmas, earning more money than you, with a cost of living crises always on their mind, etc.
> "Good enough chemistry enables cognitive treatments."
Drugs can force people to carry on with a life that's making them miserable when they have no other available options to find out why and fix it. That isn't evidence that "there is no root cause"(!). Any more than turning it off and on again can let you get on with your job, but that doesn't show there's no root cause for a program locking up.
> "then you deduce that the black cloud is produced within."
And you have a lifetime of your prior experiences affecting your mood. When you remember that your aunt hit you when you swore at the dinner table, or you saw someone slip on ice and fall over and break their wrist, or watever, every life learning experience is "the mood is produced within".