I think most depression is usually situational, such as life and environmental causes, external causes. This isn't to say that clinical depression doesn't exist; in fact, it does. It is a shame that psychiatry industry and field likes to lump the two types of depression together. Also, in popular culture and media, most of the time, when people refer the depression, the first thing that comes to mind is clinical depression and chemical imbalance theory. Again, that's bullshit because while there are cases of clinical depression and what not, I believe it is being blown out of proportion. This isn't to say that clinical depression and chemical imbalances don't exist, it's just overblown. More often than not, people seem to ignore root causes and only tackle the symptoms (both common people and doctors, mental health professionals alike). I suppose it's easier (and lazier) to just attack the symptoms rather than address the root causes for it.
To be fair, I'm not dismissing the fact that clinical depression doesn't exist (id does), but too oftenly people lump both kinds of depression into one and don't differential between the two.