There are really two kinds of depression. There's the emotional kind, which is when something bad happens and you're sad. Then there's the clinical kind, which is when your brain chemicals stop working quite right, and you think you're sad.
The world has an emotionscape, where all sorts of emotions are flying around everywhere all the time. As an empath who constantly experiences other people's emotions, I've come to understand the difference between an emotion that I myself am feeling, and an emotion from an external source. It's subtle, and the brain is naturally inclined to assign internal reasons for emotions that appear without one, which makes it all that much harder to distinguish. If anger appears, then my brain finds something to be mad about. If sadness appears, it finds reasons to be sad.
Last weekend I lost all will to do anything other than lie around staring at the wall. I didn't want to do anything else. I had no energy to write anything, or cook anything, go out for any reason, or otherwise engage in anything. Attempts to snap myself out of it by jumping into the apartment complex pool were fruitless. I played a lot of Bloons.
But I wasn't sad. I have nothing to be sad about. I just had every other symptom of a clinical depression. It went away sometime yesterday evening, possibly because I ate something specific that got my brain moving again (though I don't know for sure). Still, it was an interesting experience. It seems I've gotten so good at separating external emotions that now I can be depressed without being depressed.