Depression can be especially cruel because it doesn't affect just the depressed person, but everyone around them as well. Someone who is depressed can be very difficult and draining to deal with. What makes this illness so painful is that a depressed person's relationships can become strained--to the point where others actively avoid having anything to do with them.
This further contributes to a worsening of self-image and makes the person feel even more isolated, intensifying the illness.
Depression is an exceedingly wicked illness, preventing those it afflicts from finding treatment, and plunging them into ever-deeper isolation.
To understand just how shocking this illness is: No other disease, physical or mental, reinforces and feeds itself, as depression does.