One of the most common questions that depressed people ask is the most basic, straightforward question that can be asked about this topic. And yet, people often receive inadequate answers to this question from there psychotherapists either because the therapist does not know or because the therapist is afraid to contaminate the therapy with their own theories too early on.
Before I get into my own ideas on the subject, I will let you know that there is really no way to know for sure exactly the causes of your depression. However, within appropriate psychological evaluation by a skilled practitioner, is extremely possible to develop a good enough idea of the causes of your depression that an effective treatment plan can be designed and implemented.
One of the most difficult things for the average person to understand about their own depressive disorder is that it is generally not caused by just one thing. For example, if someone in your life that you care about passes away, that it is normal for you to experience some of the symptoms of depression as a part of the natural grieving process. But this is not depression, it is simply normal and healthy grief. In order for that grief to be transmuted into a major depressive disorder, there must be other mental health issues at play.
If you are already susceptible to becoming depressed, it would make sense for triggers such as grief, which ordinarily brings on the same symptoms as depression, to cause that susceptibility to manifest in an actual psychological disorder. I believe the same is true of physical conditions. Many physiological illnesses can cause depression, or least the same symptoms as depression, but it may be a certain susceptibility is necessary before the average person will experience a full-blown depressive episode as a result of these conditions. I think this idea will make sense to the average person, as well as the average mental health professional.
A common idea especially in psychodynamic circles is that of "overdetermination." This idea recognizes that people are basically very well adapted and able to adapt to a wide for writing situations in spite of any hardships they may have experienced. Any psychotherapist will tell you stories of patients have experienced the most difficult lives imaginable and gone on to be productive healthy and relatively happy adults. The idea of overdetermination takes this into account by positing that many different factors must come about simultaneously from a variety of directions in order to induce and mental illness. So if you are depressed, not only is it not likely to be only a single factor, it is likely to be a confluence of factors that add up to your present social and emotional condition.
So in order to cure depression, a therapist must address a wide variety of possible causes simultaneously. Truly skillful clinicians are often able to design extremely elegant interventions which address seemingly unrelated issues all at once.
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