Clinical depression is frequently misunderstood as simply an intensified version of sadness, but a core diagnostic feature is anhedonia — a loss of interest or pleasure in activities that used to matter to someone, which can occur with or without overwhelming sadness itself. Someone with depression might describe feeling flat, numb, or simply unable to bring themselves to do things they know they'd normally enjoy, rather than crying or expressing grief. This is part of why depression is so often missed, including by the person experiencing it: it doesn't always look like the media portrayal of visible sorrow.
Depression's causes are now understood as multifactorial — biological factors (genetics, neurotransmitter systems, hormonal changes), psychological factors (thought patterns, past trauma), and social factors (isolation, chronic stress, major life changes) all contribute, usually in combination rather than any single cause. This matters practically because it means effective treatment is often also multifactorial: medication addresses biological contributors, therapy addresses psychological patterns, and rebuilding social connection and routine addresses the social dimension — which is why a purely one-track approach (medication alone, or willpower alone) often underperforms combined approaches in research on treatment outcomes.