AsapSCIENCE:

While the science here is interesting and concentrates on how we think depression is linked heavily to genes, the important takeaway is that depression is a disease with a biological basis. Which is to say, it’s not just something people can “get over.” If you, or someone you know is depressed, understanding what’s really going on in the body is an important part of dealing with it.

I’m only just beginning to get a handle on my own personal issues with depression and, while this video won’t help, it is an interesting explanation of what some of the causes might be.