Good things

Depression is a hard disease to understand.
For those of us with relatively normal minds, it comes naturally after times of great duress such as losing a loved one. Depression is a normal step in greiving. For some, it becomes the last step.
Another route to depression is stress.
Depression as a result of stress is often linked with anxiety disorders. If the person is not depressed, they are constantly battling back the wave of anxiety. The hammering heart; the sense of not being able to breathe; the tingly numbness in your head that makes it impossible to think or feel anything but a desperate sense of wanting to hide.

Stress can be caused by anything really. Too much work is considered a noble reason to be stressed. That's horsecrap, frankly. There is no noble reason to be depressed and anxious.

Chronic pain causes depression. It exhausts you from deep inside.
Doctors commonly write prescriptions for anti-depressants along with pain medications. But years of chronic pain and depression can bend your mind. To avoid the anxiety attacks, the person hides in depression.

About half the population has a tendency towards depression to one degree or the other. The stages of life can cause it, too. Teenagers' violent hormonal changes can throw them into the grip of the Black Dog. -- That's what Sir Winston Churchill called it.
Those who are not depressed can't understand. Even those who have experienced it drive the memories from their mind. We humans tend to only want to remember the good things in life.