so many people just don't get it, and even more don't understand just how awful it feels - the quote below is from someone who explains it well, although JK Rowling's are good too

“What people don't understand about depression is how much it hurts. It's like your brain is convinced that it's dying and produces an acid that eats away at you from the inside, until all that's less is a scary hollowness. Your mind fills with dark thoughts; you become convinced that your friends secretly hate you, you're worthless, and then there's no hope. I never got so low as to consider ending it all, but I understand how that can happen to some people. Depression simply hurts too much.”
― Tyler Hamilton

"Depression is the most unpleasant thing I have ever experienced. . . . It is that absence of being able to envisage that you will ever be cheerful again. The absence of hope. That very deadened feeling, which is so very different from feeling sad. Sad hurts but it's a healthy feeling. It is a necessary thing to feel. Depression is very different."
— J.K. Rowling
J.K. Rowling

"It’s so difficult to describe depression to someone who’s never been there, because it’s not sadness. I know sadness. Sadness is to cry and to feel. But it’s that cold absence of feeling— that really hollowed-out feeling."
— J.K. Rowling