Surely you have also wondered how depression is a mental illness when pretty much everyone sometimes suffers from feeling down or depressed, without necessarily having a mental illness?
When sufferers or mental health professionals talk about depression, we mean clinical depression - not temporary feelings of being down or of unhappiness.
So what differentiates these from those? What - other than length of time - makes depression a true mental illness?
In my experience, clinical depression comes with a side-serve of cognitive distortion, which means thinking thoughts that just don't correspond to reality.
This is all very theoretical so far, so let me give you an example:
A person suffering from clinical depression might "think" it perfectly rational to commit suicide, as they cannot imagine a way out of how they're feeling. Yet for most sufferers of depression, the illness has both a beginning and an end, and things do get better.
Worse, a person in the depth of depression might think that it would place a terrible burden on their children to have a parent who suicides, and so think of killing his/her children too, to "spare" them.
Now, would you think that any person "in their right mind", i.e. not suffering from a mental illness, would think along those lines?
The difference between being depressed and having clinical depression is about whether we can still see our reality clearly, or whether our mind distorts them into something that a "normal" (i.e. non-ill) person would not recognise as reality.
And that is the terrifying "reality" for sufferers of depression - viewing life through a dark grey filter created by their own brains.
Well-meaning advice about getting more exercise or "looking at the bright side" just won't be helpful; they will sound to a person with depression roughly the same as "grow a third arm" - completely impossible.
Instead, try sitting with them, holding their hand, literally or metaphorically, so that at least one cognitive distortion, the one that tells them that they're all alone in this world, is proved to be wrong.