I think a common mistake is to assume that depression is defined by feeling sad. It's not at all. Probably the most prominent symptoms affect sleep and appetite. Then also cognition, interest in activities you previously found pleasure in and the amount of energy you feel you have in general. Suicidal ideation and feeling like a failure are additional defining symptoms. Actually, "depressed mood", as it's weitten in DSM 5, pretty well captures what it's about: to be pressed down. For me, depression is exactly that: a feeling on nothing. Not sadness, not happiness; just a void. Not saying everyone here have clinical depression, but my point is that depression is more than "sadness" and there is a continuum where clinical depression is at the low extreme and mania at the high, with euthymia (i.e. normal/tranquil mood) in the middle.
Another thing is that if you go depressed a long period of time you lose sense of what it is not to be depressed. As my mood shift a lot, when I look back at when I felt good during a depressive mood, I almost can't understand how my outlook could be so different. Vice versa, when I feel better, I ask myself why I got that rope and spend all my time on this forum. Then I crash again and come back here. And now I'm just tired of the constant shifts, knowing I'll crash over and over again, so even when I have my better periods, I know I'm gonna die of suicide at some point.
