Again, this is part of the problem. Your argument presupposes that a good job and a happy family would be enough for anyone, that anyone who is unhappy with that life is medically abnormal. Just because someone is superficially content does not mean that their unhappiness is inexplicable. Describing a lifestyle as "objectively wonderful" is patently absurd, based as it is on the belief that it is possible to objectively judge what sort of lifestyle would make someone else happy.

The neurochemical hypothesis is incredibly dangerous, because popular perception has drifted so far from scientific consensus. It is an incredibly weak hypothesis with only the scantiest of evidence, but is accepted as solidly proven by a great many lay people - some people in this comments thread describe it as "scientific fact". Most neuropsychiatrists and psychopharmacologists will freely admit that there is no real understanding of what depression is, let alone what causes it.

Many laypeople perceive depression as a simple chemical deficiency, much like a vitamin deficiency - give someone vitamin C and their scurvy goes away, give them SSRIs and their depression goes away. In fact, SSRIs are only marginally better than placebo and have no statistically-significant effect in the majority of patients. This belief is a strong disincentive to seek out psychosocial interventions, which are at least as effective.

Maybe good job and family isn't the only factors in leading a happy life?

Not that I agree fully that brain chemistry isn't a factor. That said, the fact that wealthier countries have higher depression rates (according to WHO) does make me think there is something non-biological involved.

Speaking from both experience and anthropological theory, I would hypothesize that wealthier countries have more depression because they are more mobile.

To start with the anthropological theory, let's take David Graeber's notion from Debt about the trauma of slavery. It's not merely being made to work by force that makes a slave. On a personal level, what traumatizes and dehumanizes a slave is that they have been ripped from their context. Everything that tied the free person to their family, their friends, their tribe/nation, their home, their religion, their culture, even their enemies, it has all been taken away.

I think whenever you take someone entirely out of their context and put them in an entirely new context, there is a chance for depression, because I think that on a deep level, depression is a phenomenon of a person being out-of-context, or at least, out of any context that can physically and mentally nourish them in the right way.