You do bring up a very good point though: sometimes depression is a mental symptom of a physical problem. Bad hormones can make you hopeless, they can take enjoyment and passion out of everything, they can completely alter your thinking and personality (I know from experience).
Which is another reason to not commit suicide when you're depressed. Even if you cannot physically imagine yourself happy, seek a doctor to check if your hormones are off. And if they can't find anything, seek another doctor (there are many many things that can go wrong with your body and cause depression, one doctor won't check them all). People's lives have been changed because they found weird holistic treatments which cured their rare hormone imbalance, which caused their depression.