I believe that depression is an evolutionary artifact. Perhaps hundreds of years ago, the men and women most likely to survive were the ones who could effectively shut down in response to environmental stressors.
So when plague was rampant, the people who responded with grim stoicism survived. I don’t mean that apathy made them more resistant to disease, just that they were less likely to panic or become hysterical and do foolish things that did expose them to risk. When governments were tyranical, depression was healthy. It kept you from doing something dumb like rebelling. Depression is the armadillo of Darwinian psychology.
Lately I am almost happy – to the extent that it is possible for me to be happy – that I am depressed. (Yogi Berra would probably appreciate that last sentence.) It feels like a warm safe fog around me. So long as the sunlight stays out I don’t have to see the devastation. I am stasis in darkness. I just don’t have the guts to put my head in the oven.