The problem with what we call depression is not the depressive mode of thought, per se. The problem is that this mode of thought gets out of control.
The depressive mode of thought is a tool, and an important one. The brutal honesty of this way of thinking allows you to analyze situations more clearly, and in so doing, it allows you to make better decisions. Being able to enter this mode of thought was likely an important step in human evolution, and we all do it from time to time, even those of us who are not "depressed".
But like most tools, the depressive mode of thought can be dangerous. That same brutal honesty, applied to one's self, does not end well: our human imperfections basically guarantee that such analysis will tear at the psyche in ways that little else can.
It is said that we should not judge others because "you weren't there; you don't know." But that doesn't work when you apply it to yourself, because you were there; you do know. And so you analyze yourself, and your judgment inevitably comes out wanting. The next question is obvious: what should you do about that? Only there's usually not enough data to answer that question, so an obvious intermediate step emerges: more analysis. And so the spiral continues.
This makes the ability to leave the depressive mode of thought just as important, from an evolutionary and health standpoint, as the ability to enter it. Pick up the tool, use it as needed, and put it down again. What we call "depression" is essentially what happens when you can't put it down.
You can probably see where I'm going with this. I think we're going about treating depression the wrong way. We try to take the edge off the mode of thought, when we should really be targeting the mechanisms that let people go into and out of it. Those mechanisms, unfortunately, are poorly-understood today.
There's also very likely to be an extremely delicate balance: you have to let people out while also allowing them back in, lest some kind of "antidepression" take hold where they can no longer pick up the tool. I don't know what such a thing would be like; it's possible that we already know it, but under another name that might or might not be linked to what we currently call depression.