Well strictly speaking, nothing in evolution happens for a reason - this isn't intellegent design, there is no agency or intentionality in evolution.
So if you didn't mean the usual metaphorical notion of, "was selected for by natural selection because it increased evolutionary fitness", what did you mean?
Well I can imagine a couple, though I don't have any particular reason to prefer them. If people who are unsuccessful in achieving status in their tribe or whatever (totally independent of their fitness otherwise) become depressed and as a result don't reproduce, children are more likely to be born to parents who can provide for them. All the suffering could just be side effects that are besides the evolutionary "goal". Sort of like how poisoning causes you to suffer a wide range of uncomfortable effects (nausea, vertigo), when perhaps the only real evolutionary "goal was to make you vomit the poison.
I think the prior poster may have been making a point more along the lines that depression is a downside of having benefits like neural plasticity or something, not that this disease state is necessarily beneficial in itself. More like how the downside of having high-performance tires is poor rain traction or something.
How does that constitute an evolutionary model? The genes for depression reach a dead end in the people who didn't reproduce; they aren't being passed on in the children of the people who did. Is this a kin or group selection model?
I assume you're talking about the first one? Having the gene for depression here would not always cause the individual to be depressed. Actually that would hold for both cases.
The trait is passed on because it benefits the species. You have to consider more than individuals in the evolution of social animals. In various circumstances some organisms will eat their own offspring. Obviously the "infanticide gene" if considered alone, would not make evolutionary sense, yet there it is.
There is no benefit to a gene in benefitting the species per se. A trait that increases the size of the gene pool while decreasing its own absolute prevalence will die out.
Group selection is a possible mechanism in social animals, but it requires the group to be strongly interrelated relative to the rest of the species, and it relies on the group being able to expand at the expense of the rest of the species.
Infanticide makes perfect sense; offspring are only related 50% to the parent, so if resources are less than 2x as effective in the bodies of offspring compared to once they have been digested by the parent, of course they should be consumed. The genes of the offspring won't agree, of course; for them the calculation is reversed.
I didn't say the trait would decrease its own overall prevalence. The trait wasn't simply to be depressed if get the genes, it was a predisposition to being depressed if circumstances not good for reproduction. If resources are limited, it would simply ensure that more successful members of a species are the ones to reproduce and get them. Same as in your infanticide calculation.
But what else could it be? You can’t just say that something has an evolutionary cause without also implying that the trait is for the purpose of survivability, which we associate with the notion of “good”.
So if you didn't mean the usual metaphorical notion of, "was selected for by natural selection because it increased evolutionary fitness", what did you mean?