Levels of explanation
Almost all behavior is caused by many factors acting at different levels of explanation.
These multiple causes are not independent of one another (e.g when one cause is present other causes tend to be present as well)
… some people may be depressed because of biological imbalances in neurotransmitters in their brain. The resulting depression may lead them to act more negatively toward other people around them, which then leads those other people to respond more negatively to them, which then increases their depression. As a result, the biological determinants of depression become intertwined with the social responses of other people, making it difficult to disentangle the effects of each cause.