There are aspects of living in this world, today, that will drive you nuts. If you're not nuts to begin with.
I think everyone has some form of hypochondria - some degree. You feel the odd pain here or there and you find yourself running your mind through the list of ailments which correspond symptomatically. You can't help it. Everyone does it to some degree or another.
Then there's Syndromes and Conditions - things like Ehler's Danlos or Autism and Aspergers. Things that, although physical and dramatic at times, have no locus - no physical deformity to be operated on or adjusted, no defective brain part to be excised or jolted with electricity.
And so many times these things we have are "invisible" - they don't have an outward manifestation like Chicken Pox or Acne. They hide - like Depression.
Depression, like all of the above named things, happens in "degrees". Sometimes you just don't feel like you have the energy to do the things you know you should. Or the ambition. So you watch TV or play around on the computer.
Or sometimes you feel like nothing you do is good enough and nothing ever seems to go right and what's the use of even trying. You keep trying, but it just never seems to be enough.
And sometimes you can't even get out of bed.