If I had to define a major depression in a single sentence, I would describe it as a "genetic/neurochemical disorder requiring a strong environmental trigger whose characteristic manifestation is an inability to appreciate sunsets.
On an incredibly simplistic level, you can think of depression as occurring when your cortex thinks an abstract thought and manages to convince the rest of the brain that this is as real as a physical stressor.
Depression is not generalized pessimism, but pessimism specific to the effects of one's own skilled action.
Most people who do a lot of exercise, particularly in the form of competitive athletics, have unneurotic, extraverted, optimistic personalities to begin with. (Marathon runners are exceptions to this.)
