Every single time I have experienced episodes of depression, I have been able to trace the depression to environmental causes/stressors. In most cases I had at least an inkling of the cause at the time I was suffering, but there were reasons why correctly identifying the source of suffering would be problematic. In all cases the causal relationship became obvious in hindsight, once the environment had changed.
For example:
When you're feeling depressed because you're a child being abused by your parents, consciously acknowledging that your parents are responsible for your suffering would pose an existential risk. The "correct" thing to do in this situation to end the depression would be to leave the abusive situation. But when you depend on your abusers for your survival, that's not really an option. Therefore the way that most kids survive these situations is by suppressing their true feelings, pretending that everything is okay, and exhibiting symptoms of depression/anxiety/oppositional defiant/name your disorder. This isn't done consciously, it just happens.
The same thing happens as adults, replace the abusive parent with a spouse/boss. The key factor is that removing oneself from the harmful environment would entail significant cost to one's social life, career, or ability to support oneself. Therefore you stay in the situation, and eventually the pain you are continually experiencing manifests itself in depression.
For example:
When you're feeling depressed because you're a child being abused by your parents, consciously acknowledging that your parents are responsible for your suffering would pose an existential risk. The "correct" thing to do in this situation to end the depression would be to leave the abusive situation. But when you depend on your abusers for your survival, that's not really an option. Therefore the way that most kids survive these situations is by suppressing their true feelings, pretending that everything is okay, and exhibiting symptoms of depression/anxiety/oppositional defiant/name your disorder. This isn't done consciously, it just happens.
The same thing happens as adults, replace the abusive parent with a spouse/boss. The key factor is that removing oneself from the harmful environment would entail significant cost to one's social life, career, or ability to support oneself. Therefore you stay in the situation, and eventually the pain you are continually experiencing manifests itself in depression.