We learn to live with pain in varying degrees daily, or over longer periods of time, and we are more often than not mercifully free of it. When we endure severe discomfort of a physical nature our conditioning has taught us since childhood to make accommodations to the pain's demands-to accept it, whether pluckily or whimpering and complaining, according to our personal degree of stoicism, but in any case to accept it. Except in intractable terminal pain, there is almost always some form of relief; we look forward to that alleviation, whether it be through sleep or Tylenol or self-hypnosis or a change of posture or, most often, through the body's capacity for healing itself, and we embrace this eventual respite as the natural reward we receive for having been, temporarily, such good sports and doughty sufferers, such optimistic cheerleaders for life at heart.
In depression this faith in deliverance, in ultimate restoration, is absent. The pain is unrelenting, and what makes the condition intolerable is the foreknowledge that no remedy will come-not in a day, an hour, a month or a minute. If there is mild relief, one knows that it is only temporary; more pain will follow. It is hopelessness even more than pain that crushes the soul. So decision-making of daily life involves not, as in normal affairs, shifting from one annoying situation to another less annoying-or from discomfort to relative comfort, or from boredom to activity-but moving from pain to pain. One does not abandon, even briefly, one's bed of nails, but is attached to it wherever one goes. And this results in a striking experience-one which I have called, borrowing military terminology, the situation of the walking wounded. For in virtually any other serious sickness, a patient who felt similar devastation would be lying flat in bed, possibly sedated and hooked up to the tubes and wires of life-support systems, but at the very least in a posture of repose and in an isolated setting. His invalidism would be necessary, unquestioned and honorably attained. However, the sufferer from depression has no such option and therefore finds himself, like a walking casualty of war, thrust into the most intolerable social and family situations. There he must, despite the anguish devouring his brain, present a face approximating the one that is associated with ordinary events and companionship. He must try to utter small talk, and be responsive to questions, and knowingly nod and frown and, God help him, even smile. But it is a fierce trial attempting to speak a few simple words.

(William Styron "Darkness Visible: A Memoir of Madness")