Of course they are. And there's a lot of variability in diagnosis between practitioners. Some even say that depression isn't really a disease at all.
There is an excellent book on this topic that I cannot recommend highly enough:
Title: "Manufacturing Depression: The Secret History of a Modern Disease"
Quote: (about Congitive-behavioral therapy) “a method of indoctrination into the pieties of American optimism, an ideology as much as a medical treatment.”
Here's a discussion of the book (and topic) at the New Yorker entitled "Head Case": http://www.newyorker.com/arts/critics/atlarge/2010/03/01/100...