I think this is one of the thorniest questions regarding depression. It appears to have biological, environmental (motivational) and cognitive components.
The biological component can be seen in the most popular hypothesis of how depression works. This hypothesis is that depression is caused by a shortage of the neurotransmitterserotonin and possibly others. This is why the most common treatment for depression is with a selective serotonin reuptake inhibitor (SSRI).
This begs the question, why isn’t there enough serotonin? Some individuals such as myself have alleles (genetic mutations) that cripple the body’s ability to create L-methylfolatefrom folic acid. L-methylfolate is key in the production of neurotransmitters. The treatment for this is taking L-methylfolate capsules. It didn’t work for me, so I believe this picture to be incomplete.
Are there people with the biological factors for depression, who never get depressed? The reason I ask this question is because depressive episodes are usually triggered by something in the environment. If you never encounter a trigger, can you avoid depression? An example of a trigger is a hostile neighbor. This is what I think you mean by motivation. However, the depression is out of proportion with the trigger. A healthy person may only feel annoyed when encountering the same event.
One treatment is to find an environment that doesn’t trigger the depressive, such as moving to another neighborhood (away from the hostile neighbor). Another one is improving one’s social skills and relationships through interpersonal psychotherapy(IPT). In this case, the depressive may learn how to peacefully coexist with the hostile neighbor.
A special case of the environmental factor is seasonal affective disorder (SAD). This is caused by a lack of sunlight. In this case, one modifies the environment by using a light box or changing all of your light bulbs to full-spectrum light bulb or one might change the environment by moving to a sunny location.
Then there are the negative thoughts that can result in negative feelings and maladaptive behaviors which result in more negative thoughts. This continues in a vicious circle until one is depressed. This is treated using cognitive behavioral therapy (CBT). Is the vicious circle resulting in depression triggered by something in the environment. I suspect so, but this is outside my sphere of knowledge.
Do some people only have one of the above factors? Some people are successfully treated with an SSRI. Others are successfully treated with CBT or IPT. This implies that only one factor was operating.
Then there are the people who need both an SSRI and CBT or IPT. I expect that they have two of the factors.
For those people, who are successfully treated with SSRI’s, CBT or both, was there a trigger? If not, there was no motive for their depression.
Then there are those who are on a cocktail of medications, had spent years in therapy, use a light box, have had ECT’s, changed jobs and living situations etc. and don’t seem to respond to anything. That’s me. Perhaps, there is a forth factor we haven’t yet identified.
©2016 Stephen L. Martin
Painting: Summer Interior by Edward Hopper