Depression is a terrible illness because it cannot be physically seen, but its effects are felt by the sufferer and are potentially devastating.
A very good friend of mine suffered with this for a number of years, and two years ago he took his own life. No one, even his wife, did not realize just how ill he was. I, among others tried to help as far as we could, but he always minimized his suffering, and just said, with a smile on his face, that we were kind for caring about him, but that he would be OK.
Sufferers need help, beit from a professional or someone who excels in counselling of some kind, who understands the illness and how they are really feeling, so the right kind of treatment can be given to help them cope with the illness, as I don't believe that depression is completely curable at the moment.
John