I'm not sure I agree with that second part. The characteristics of depression are prolonged sadness and overreaction where others tend to move on after some time, like after a death, for example, if we're talking about depression due to specific circumstances. I think such a thing is also possible, but that hasn't been my personal experience. I started feeling depressed in my teens and I'm 34 now and still feeling it. No doubt my circumstances have changed many times over the years and no matter what they were the depression hasn't changed. But again, I think both are possible.
On the other hand, maybe you're right. IDK. Maybe the criteria for depression are misguided.
I also think, that it depends on what kind or type, of depression you have: say; on the more severe or as you say, prolonged side (major) or whatnot. Some people, let's say they're more on the other end, or even in the middle. They can respond favorably and therapeutically to medications and such (psycho therapy). Then they can either function, with or in some cases--after a while--without those devices, or drugs. However, when you come to the more chronic and acute cases.. Such as that you or I have. I believe it is a completely different beast. Or can be, especially as you say, when it doesn't seem to be any logical or practical & tangible explanation for it (I lost such & such out of my life, I got fired, I went to jail)... It could be genetic, it could be some combination therein. And even more so, if you do not respond very well to antidepressants, or other medications. Then it can be an even greater roller coaster ride. Or mixed bag of experiences. When it comes to - successes & failures (one's). Now, are there/or have there ever been in the past; periods where this has (your depression) gone into remission... completely? For me, you see I have. Or it has. But always to return. And with each crushing or devastating & debilitating period or one (episode, let's say). They get harder, and longer, and are more difficult to resolve. What began as months at a time, years & decades ago. Now take s more like years, to be frank~ / So there may be no real good talking point style of answer or reason, that you can give someone. Who asks you that question then. Even to answer for yourself. You just sort of know it, intuitively, and from within the gut. I'd suppose, or at least I do. Again depression is a disease of the mind, or brain. An organ. Structure, and function. Trying to figure out the "why," may be futile. Even if it would satisfy some underlying suspicion, or curiosity or concern, even. Help us to sleep better at night. I don't know. I can barely keep my eyes open, and my brain or mind, on. Have fun trying to analyze or dissect this. Might take a neurosurgeon, of significant skill, and dazzlying wonder & appeal. To psychogenically solve our ills. On e that doesn't (then) exist, in other words* ^~> ;^) this is (also) why people say (other hard scientists/or pure) that psychology is a very 'gray/grey,' science. Anyway, until better & more proven methods come along. And are developed. Such as a blood test to tell us: Hey?!? Guess what, you've got such & such. Congratulations! :-D (this, is waht we're stuck w/I guess). Again, if you're on the more severe end of one of these mentla illnesses. IN my oopinion. . . (^: