I think the worst thing about depression is the insidiousness of its symptoms; all of which further contribute to the ever deepening spiral into the dark abyss of meaninglessness.
When depressed, people tend to isolate themselves, staying at home with the curtains drawn, the room dark, not answering the phone, not answering the door and vegging out in front of the television or sleeping all day. They often don't eat, or are drawn to sugar and carbs. Their thoughts often become as dark as the room their sitting in.
"What's the point?"
"Nothing matters so why bother?"
"Why does everything have to be so hard?"
This leads to more inertia, more isolating, less socializing. Which in turn leads to darker thoughts.
The "cure" would be to do the opposite of what one feels like doing. But this would require energy you don't have and motivation that is non-existent. So you spiral further and further down until the bills are overdue, you've lost your job, friends no longer call and family is tired of you. Your thoughts become even darker to the point that suicide starts to look like a good solution to the problem.
This is when medication can be useful, but who wants to take a pill everyday? And the medication comes with side effects which are sometimes as bad as the depression.
That's why it's important to keep people around you who will give you honest, but constructive, criticism. People who will tell you when they see you isolating or getting really negative. It's easier to head it off when it first begins than to try to drag yourself out of it later. I've written a lot about the ways to fight depression, so I won't repeat them. You can find those articles by clicking "Depression" in the Categories section to the right or by clicking here.