The worst thing about depression is knowing that at one point, you were different than this. You know what's it's like to be excited and generally optimistic. You know that life for many people is, no matter how much they complain, better.
You know that you're depressed and that this is no way to live.
But the current is too strong to swim against even if you tried. You see life slip away and fade, leaving only memories of a time before the monster.
Soon, you don't see a reason to go back. We all know depression-free is more habitable, but why take back what you have lost, if you can even find it? And all that effort? Blech.
It is easier to simply float through existence, bleak and gloomy as it may be.
And then you begin to forget what it was like to not only cry, but to laugh. And that is when you have lost yourself in the folds of darkness.