I didn't make myself clear. I'm not dismissing what the actual book says, simply disputing your apparent claim that the book has a checklist of what constitutes as a valid reason for triggering a depression. I very much doubt it goes like "losing one pet: mild sadness, losing two pets: strong sadness, losing four to five pets: depression available." In a way you could say depression is an extremely disproportional reaction to something. Sure, there are cases where it's considered "spontaneous" but I do wonder if those are in fact completely out of the blue for no reason whatsoever or simply seen like that because of such a tiny and trivial trigger that the link between cause and effect just isn't made. As you said, depression isn't a rational reaction, and as such it's not limited by what you consider "valid" for causing it. People handle your "bereavement" in different ways, just like they get affected by different things. What will make some people cry will get just a shrug from others. When you put that together with the disproportionate reaction aspect I see on depression, you just can't dismiss losing a job or a break up as possible triggers, because the trigger doesn't has to make sense to you. I mean, even for the depressive person it often doesn't makes sense after the episode is over.
But back to the initial thing, I do consider being a clinically depressive person and being drunk different things. As I said, I see the first as "pure," because it's simply the person's brain working the way it is. I don't see it as "defective," just as working in a non-standard way, but still pure regarding what it is. You're born like that, while you aren't born a drunk (I'm not going to get into cases of mothers who take drugs and the child is born already addicted). If you go picking and choosing what aspect to analyze and compare between depression and drunkenness you might as well say both are the same because on both you are liable to do stupid stuff, but the point of view I used is that on one your brain is under the influence of external substances that cause that state, while on the other your brain is just being itself, working with it's peculiar chemistry.