October 23, 2019:
When I encounter someone describing their sadness as "depression" I frequently scratch my head in puzzlement. I'm not sure what they mean. For me, at least, sadness and depression are distinct phenomena easy to distinguish.
Sadness has a visibly tangible triggering cause and is objectively reasonable. It's an outcome. I'm sad from the breakup of a relationship, death of a parent, winding-down of an old friendship. Sadness and grief are related, where grief is acute and immediate while sadness is more generalized, less in-my-face. Sadness has reasons, and I know what they are.
Depression is uncaused. Rationally I know it's caused by chemicals and hormones going apeshit in my brain. But there is no triggering life event in the sense of the death of a beloved pet. That's grief, it's a response to a real-world event, and it's objectively reasonable.
Depression just is. It's a lack of energy, an inability to find a reason to care. When my beloved cat dies and I don't feel grief, I just feel numb and exhausted and wish I could step away from the reality of the world, that's depression.
Maybe it's different for these people who use "sadness" and "depression" interchangeably. I don't know.
Or perhaps, they've never experienced depression, only sadness, so they can't suss the diff. I don't know that, either.