Even if your particular depression does include sadness, it’ll only be one of many other symptoms. The others might be much more painful and salient for you than the sadness is. Some people can’t sleep, others gain weight, some think constantly about death, others can’t concentrate or remember anything. Many lose interest in sex, or food, or both. Almost everyone, it seems, experiences a crushing fatigue in which your limbs feel like stone and no amount of sleep ever helps. Then there are headaches, stomachaches, and so on.
So, depression doesn’t necessarily mean sadness to us. (And a gentle reminder to non-depressed folks: being sad doesn’t mean you’re “depressed,” either.)
Depression is not sadness; it’s an illness that often, though not always, involves sadness. No amount of happy things will make a depressed person spontaneously recover, and, usually, no amount of sad things will make a well-adjusted person with good mental health suddenly develop depression. (Grief, of course, is another matter.) And sadness, on its own, does not cause suicide.
[…]People don’t kill themselves because they’re sad. They kill themselves because they have an illness that, among other things, makes them feel sad. It also makes them feel like their life is worthless, like they’re a burden to others, like death would be easier, and all the other beliefs that lead people down the path to suicide.
There is a tendency, I think, to assume that people are depressed because they are sad. A better way to look at it is that people are sad because they are depressed. That’s why, even if we could “turn that frown upside down!” and “just look on the sunny side!” for your benefit, it would do absolutely no good. The depression would still be there, but in a different form.”
— Miriam Mogilevsky, Depression Is Not Sadness: Junior Seau and Public Discourse on Mental Illness (via slothgrrl)