In the past, the story behind depression was the brain lack or imbalance of a neurotransmitter called serotonin; being a therapist now & getting to know the depressed persons more tell me a different story- it's a whole body response to protect them from being hurt again & again.
How? By not having hope (feeling hopeless) & refused to be helped (feeling helpless), there's no hope therefore no expectations, no expectations mean no disappointment, no disappointment mean no hurt.
Is this a conscious decision? No.
This happened at a much deeper subconscious level that the person may not even aware about it. Perhaps, this is the way the body response to a perceived extreme life-threatening situation, simply to shut-down.
Hence, depression is not about sad or low mood, it's about not feeling anything/numbness so that one can't be hurt by unpleasant emotions anymore.
However the downside is that one would not experience joy or pleasant feelings either.
Imagine how would it like; to look and not seeing, to hear and not listening, to touch and not feeling. That’s how a person suffers from depression.
Depression keeps you from hurts but also keeps you from living a colorful life, depressed people can see colour but not feeling it, hence the metaphor of seeing only black and white .