The categorization of depression is that it's an emotional disorder. This is not an inaccurate description, as depression is marked by a dysfunctional mental outlook. There is a difference as well in the mental outlooks of people who are and are not depressed. Depression, however, is by no means confined to only being a psychological process.
The reality is that most of depression's symptoms are physical in nature, not psychological. Some of the more common symptoms of depression include excessive sleep patterns or sleeping too little, weight gain or weight loss, lack of energy, emotional outbreaks, and other symptoms as well. Considering this symptom list, being overly emotional is the only symptom that might be seen as mostly psychological in nature, but even that presents in a physical way through crying or hostility or whatever.
The labeling of depression as a psychological problem, a problem of the mind, equates it with mental instability or weakness in the minds of some. Men in particular seem to be especially sensitive to being labeled with some form of mental or emotional disorder. On its face this type of resistance may not seem overly problematic, but it can become quite a serious issue.
Resistance to even admitting to possibly being depressed is naturally going to lead to treatment resistance as well. Depression has been known to clear independent of any treatment. This is perhaps particularly the case in situations of meaningful loss, such as a death or a relationship coming to an end. But bouts of depression that aren't triggered by a significant incident, or depression brought on by a heavily traumatic event, can become chronic depression. Depression that has become chronic almost certainly requires treatment, and without it can lead to significant emotional distress and in some cases even a suicide attempt.
Emphasizing the physical nature of depression can diffuse the stigma of a depression suggestion or diagnosis. This may be the reason that depression is sometimes attributed to a chemical imbalance in the brain. While some clinicians don't like this description, there's significant evidence that the brains of depressives do have a different make up than the brains of people who aren't depressed. In other words, the brain is changed when depression sets in. This knowledge is what has sparked the development of a host of depression treatment drugs over the last twenty years or so, each designed to "fix", in its own way, the depressed brain.
So depression changes the brain, and could even be said to imbalance the brain. The makeup of the brain being somehow changed would seem to define a problem that's physical in nature, so depression may be a mostly physical condition after all.