The depressed brain isn't a machine that broke that just needs a replacement part. The brain is a machine that in the course of its operation changes the way its made. Depression is a dark anti-pattern in which the brain begins to disassemble itself. What you think changes your brain and changes in your brain affect what you think.
The only way to "fix" it is to build again what is lost. Can a drug help? Sometimes, but building is long and hard. A depressed brain is like a house thats been neglected for 20 years. It needs lengthy remodeling, not a new paint job.
> Depression is a dark anti-pattern in which the brain begins to disassemble itself.
I'd agree that it's an anti-pattern, but rather than the brain disassembling itself I'd say the brain assumes a very stable, self-reinforcing thought model which rewards negativity or anhedonism. I describe my experiences with depression above, so I won't repeat them here, but I'm convinced that the only "cure" is finding a way to disrupt this feedback loop. Like others suggest sometimes it's drugs, sometimes it's traditional psychotherapy. Believe it or not there's been a lot of recent success with shock therapy treatments for various forms of deep depression - I'd assume because it disrupts this thought model.
"What I cannot create I do not understand."
-Feynman
Personally I think we won't fully understand depression and other "thought model" problems until we have an accurate general model of thought processes in the brain and the ability to manipulate them. In the mean time I think that the tools we have are blunt instruments.
The thing about the brain disassembling itself was meant literally. Cells actually degrade and die during depression, and have to be grown back later as you get better.
Except anti-depressants can start working in ~2 weeks. SSRIs do something to serotonin levels/receptor levels. If your brain is literally degrading itself, that could just be a side effect of the lethargy, low stimulus, etc. that a depressed person is subjected to. When someone takes MDMA, I don't think they suddenly grow a bunch of connections; it just jacks up the level of serotonin to an extreme degree.
We don't understand them well and SSRIs don't work for everyone, but I don't think your cell-death theory is an open and shut case at all.
The cell death is a feedback loop. I've taken lithium carbonate when I got depressed. The fact is that however much you may be fighting the depression on your own, the chemical aid will not "cure" you, nor "make you happy and right", but it is an immense aid to breaking the neurochemical feedback loops and thus to letting you run your own brain again.
The only way to "fix" it is to build again what is lost. Can a drug help? Sometimes, but building is long and hard. A depressed brain is like a house thats been neglected for 20 years. It needs lengthy remodeling, not a new paint job.