Have you ever tried to describe colour to someone who’s been blind since birth? Go ahead, try it for yourself; pick any colour and describe it. It’s not easy is it. Maybe it’s possible but, at the end of your effort, even if the blind person says, “aaahhh, I get it now, thank you, I now understand what yellow looks like”, you’ll never be certain that they really do.
Describing depression to someone who’s never been depressed is harder. Imagine a blind person trying to describe to you what utter blackness looks like. You won’t be able to imagine it.