Andrew Solomon:

Depression is the flaw in love. To be creatures who love, we must be creatures who can despair at what we lose, and depression is the mechanism of that despair. When it comes, it degrades one’s self and ultimately eclipses the capacity to give or receive affection. It is the aloneness within us made manifest, and it destroys not only connection to others but also the ability to be peacefully alone with oneself. Love, though it is no prophylactic against depression, is what cushions the mind and protects it from itself. Meditations and psychotherapy can renew that protection, making it easier to love and be loved, which is why they work. In good spirits, some love themselves and some love others and some love work and some love God: any of these passions can furnish that vital sense of purpose that is the opposite of depression.
…It is too often the quality of happiness that you feel at every moment its fragility, while depression seems while you are in it to be a state that will never pass. Even if you accept that moods change, that whatever you feel today will be different tomorrow, you cannot relax into happiness as you can into sadness. For me, sadness has always been and still is a more powerful feeling; and if that is not a universal experience, perhaps it is the base from which depression grows. I hated being depressed, but it was also in depression that I learned my own acreage, the full extent of my soul. When I am happy, I feel slightly distracted by happiness, as though it fails to use some part of my mind and brain that wants the exercise. Depression is something to do. My grasp tightens and becomes acute in moments of loss: I can see the beauty of glass objects fully at the moment when they slip from my hand to the floor. “We find pleasure much less pleasurable, pain much more painful than we had anticipated,” Schopenhauer wrote. “We require at all times a certain quantity of care or sorrow or want, as a ship requires ballast, to keep on a straight course.”