We all know the major state of minds that are usually and constantly associated with depression, there are mostly sadness and any other down feelings that puts a person on a solitude mood, without the feeling of wanting to associate or being happy. If we draw from this, then we'd realize that depression can easily be spotted when it takes form in a person. And if we observe carefully, then we'd also be able to figure out the actual cause of the said person's state of depression, even though most people think the causes of depression is unknown.
Some people are natural when it comes to depression. Am talking about certain people who are more than just introverts. They are introverts to a point beyond our comprehending. These kinds of people are likely to be sociopaths or psychopaths. They were either born that way or nurtured in away that affected their humane personality, usually through trauma caused over a long period of time. With this type of people, we can simply say that depression in them is as a result of traits inherited through genes, or a medical condition inflicted from years of abuse. These types of depression are often difficult to treat, or even untreatable because it's strong that people suffering from it fail to know the difference between reality and what's not.
On the other hand, they are many normal and upright people who often slip into depression once in a while. This type of depression in my opinion is a minor state of depression. Minor because almost everyone can experience it. It is caused mostly as a result of events or situations happening around our lives which flares our emotions, a bad event will most definitely cause sadness, and if unchecked, can very well lead to depression. This type of depression can very well be treated and a person can overcome it in a very short while.
Another form of depression which can be easily spotted in a person is the state of mind induced through constant and regular use of abusive drugs. Drugs have a strong way of controlling and staring peoples emotions even when taken on prescribed dosage. When it becomes abused, it reacts by multiplying the side effects which can leave the abuser in a solitude state not far from depression. This type of depression can also be treated since its induced through the use of drugs. Rehabilitation will be the way out.
Over all, i think there are many other causes of depression and if studied carefully, the causes can be understood and a possible solution profered.