Nobody likes to talk about it because most have experienced a short period of depression sometime or another, and just the thought of that experience scares the life out of them. Depression is sort of like a demon. When it possesses you it is in control, and you don't even know it's there. When you're suffering from depression, you're most likely the only one who can't see the change in your attitude. Everyone around you can recognize a dark cloud surrounding you, but convincing you there's something wrong will only invite an argument.
Stress is the major contributor.
There are other causes, but stress leads the list. And even though we wonder how our great-grandparents survived without modern technology, we have many more stresses in our time than they had. Modern technology has sped everything up. It takes a lot more energy today to keep up than it did 40 years ago. Change is another stress producer.