The Causes of
There is no one answer but several possibilities. More than one possibility can be involved at the same time.
One mechanism is that of a "biological depression." This means that a person is depressed because of a biochemical imbalance of some sort. Usually this involves the brain and various neurotransmitters. Neurotransmitters are chemicals that help different areas of the brain communicate with each other. If these chemicals are low then miscommunication can occur and depression may be the result.
It is similar to diabetes in which a physical malfunction of the pancreas results in the need for a medication that corrects the imbalance. Insulin is often needed and with insulin the body is placed back into a state of regulation. Usually there is a continued need for the medication so the person is enabled to lead a normal life. Lifestyle changes must also be made such as adopting a new diet, following an exercise program, and learning to manage stress.
If there is a biochemical component to your depression you may need medication to help you overcome it. When in a deep depression the medication can help "jump start" the recovery process so you can then do the other work of healing.
Some people but not everyone have a significant biochemical component to their depression. Usually other factors are involved as well.
Some people learn to be depressed.
It may be that you were grew up in a family where everyone was depressed and think that such moods are normal. Just as you would be angry if you were raised in an angry family, or anxious if you come from an anxious family, so you may learn to be depressed if your family was depressed.
On the other hand it might also be that your family was normal but busy. As a child if you were playing quietly then your parents might think, "Good. Dont disturb her. Let her play." In essence they would ignore you if you were good. Later, if you were crying and upset because your toy broke, they might rush over to see what was wrong. Whether they meant to or not your parents just taught you a rule about life. It is that to get attention in this family you must be in distress. This may be the first step in learning a life style of depression as attention getting behavior.
Many people think themselves into depression.
Develop a habit of negative thinking, always make things worse than they are, and you can lead yourself down the road to depression.
Thinking: A Common Pathway
So, there are at least four pathways to depression, and they often act together. If you are depressed then you need to evaluate all of these factors in your life.
In your evaluation you will most likely find that thinking is always involved, and that it is a common pathway to depression. The good news is that you can control your thinking. You can choose what you tell yourself. Learning to control the pathway of thinking can give you control over your depression.