I felt a funeral in my brain, and mourners to and fro kept treading, treading till I felt that sense was breaking through. And when they all were seated, a service, like a drum, kept beating, beating, till I felt my mind was going numb. And then I heard them lift a box and creak across my soul with those same boots of lead again, then space began to toll, as if the heavens were a bell and being were an ear, and I, and silence, some strange race wrecked, solitary, here. Just then, a plank in reason broke, and I fell down and down and hit a world at every plunge, and finished knowing then.
~ Emily Dickinson
It's a strange poverty of the English language, and indeed of many other languages, that we use this same word, depression, to describe how a kid feels when it rains on his birthday, and to describe how somebody feels the minute before they commit suicide.
There are three things people tend to confuse: depression, grief and sadness.
Grief is explicitly reactive. If you have a loss and you feel incredibly unhappy, and then, six months later, you are still deeply sad, but you're functioning a little better, it's probably grief, and it will probably ultimately resolve itself in some measure. If you experience a catastrophic loss, and you feel terrible, and six months later you can barely function at all, then it's probably a depression that was triggered by the catastrophic circumstances.
People think of depression as being just sadness. It's much, much too much sadness, much too much grief at far too slight a cause.

In a way depression is continuous with normal sadness. There is a certain amount of continuity, but it's the same way there's continuity between having an iron fence outside your house that gets a little orange dust spot that you have to sand off and do a little repainting, and what happens if you leave the house for 100 years and it rusts through until it's only a pile of orange dust. And it's that orange dust spot, that orange dust problem, that's the one that therapists set out to resolve.
The opposite of depression is not happiness, but vitality. Depression is demonstrated in Eeyore's Syndrome - the "Oh, well I guess we need rain too..." flat-line, monotone, lack of expression, lack of vitality, way of thinking about yourself and the world around you. It gets down to the point you just don't care - about anything; what (or if) to eat, which team wins, whether or not you have friends, and whether or not you live. It just doesn't matter. Period.
When you are depressed, you don't see that you've put on a gray veil and are seeing the world through the haze of a bad mood. You think that the veil has been taken away, the veil of happiness, and that now you're seeing truth. It's easier to help schizophrenics who perceive that there's something foreign inside of them that needs to be exorcised, but it's difficult with depressives, because we believe we are seeing the truth.
But the truth lies.
People will say, "No one loves me." And you say, "I love you, your wife loves you, your mother loves you." You can answer that one pretty readily, at least for most people. But people who are depressed will also say, "No matter what we do, we're all just going to die in the end." A lot of the time, what they are expressing is not illness, but insight, and one comes to think what's really extraordinary is that most of us know about those existential questions and they don't distract us very much.
Depression is so exhausting. It takes up so much of your time and energy, and silence about it, it really does make the depression worse, but the silence is necessary to avoid the stigma surrounding depression.
In Rwanda, East Africa,they have some rituals that rely upon working towards vitality,but they had a lot of trouble with Western mental health workers, especially the ones who came right after the genocide.
As described by a natural healer...
Well, they would do this bizarre thing. They didn't take people out in the sunshine where you begin to feel better. They didn't include drumming or music to get people's blood going. They didn't involve the whole community. They didn't externalize the depression as an invasive spirit. Instead what they did was they took people one at a time into dingy little rooms and had them talk for an hour about bad things that had happened to them.We had to ask them to leave the country.
Depression is broadly perceived to be a modern, Western, middle-class thing, but it is a human condition that has probably been around since "Lucy" although known by many different names and shown in countless ways and means in every country and culture in the planet.
Depression sucks.