How do you describe depression to those who haven't experienced it? No doubt many would say that a sufferer feels very sad, unhappy, miserable, etc. Perhaps the more enlightened layperson would talk about the lack of any emotion except negative ones, how it can have cognative and physiological effects. That it can effect speech and movement as well patterns of sleep, how much energy a person can exert in day to day living. But it's all just words. To understand depression you can only experience it but perhaps I can describe it another way.
Let's imagine, if you'll indulge me for a moment, you're sitting in a dentist's chair. You're about to undergo a very painful and very long dental procedure. Your mouth is clamped open and the dentist is about to present the drill to one of your teeth. You feel panic rising in you, barely controlled and red raw, is made worse because of your helplessness. You hear the high pitched whining of the drill as it's just a fraction of an inch from your tooth. That fear filled moment of anticipation, I want you to hold it in your imagination, in all it's pit of the stomach queasiness and heart racing discomfort. Now, in your imagination, remove the drill and the dentist, the chair and the clamps. Take it all away, but leave the fear and the helplessness. Take that feeling and stretch it out for days, months and years until it becomes a physical pain. Existing becomes painful, the world becomes your enemy.
Some would rather die than carry on feeling that way, day after day. Others try to muddle on as best they can but they are disabled, as much as a wheelchair user or a sense impaired person. Depression is insidious and evil and it can kill.