Seriously, I think Depression is like any other pleasure/pain physical or emotional feedback mechanisms that's critical for our survival.
My view is that the loss of interest in normally pleasurable activities, sense of withdrawal, etc is nature's way of taking your attention away from the outside superficial world and onto yourself, your desires, your expectations, and where your life is off track. Your subconscious is in effect telling you "This is important! You need to forget about your habitual world and change what you gotta change - no matter how drastically. Your life is seriously out of wack with your expectations and you can't ignore it or be OK with it."
Of course big pharma would rather tell you that it's some sort of shameful chemical imbalance and mental disorder that happens to require their expensive and dangerous chemicals for life. Calling Depression a disease instead of a natural feedback/survival mechanism nets them easy $trillions a year.
So I mean it when I say "my life is going so off the tracks it would be wrong for me NOT to be depressed". It should be a natural reaction and result of your life being out of sync with your expectations. Depression is nothing to be ashamed of and in fact something that can be irreplaceably helpful if you're tough enough to take it on, rather than hiding from it with narcotics, alcohol, anti-depressants, etc.
In a sense, fasting does something similar. Your body stops processing the usual incoming foods, toxins and nutrition, and goes into a process of shutting down the metabolism and concentrating on self regulating (cleansing). Why sadness/grief/depression and fasting are so complimentary I have no idea. But they do have a similar mechanism for sure.
And from what I've seen and read, fasting is probably the most beneficial remedy for depression of all. Depression gives you the pain and motivation to change, and fasting gives you the clarity and break from the past to make necessary, radical, and life altering adjustments.