Depression is not "sadness without cause". It is a mental condition thought by some to be best described as a chemical imbalance in the brain, and often does ha

Depression is not "sadness without cause". It is a mental condition thought by some to be best described as a chemical imbalance in the brain, and often does have a very clear cause. Head injuries, drugs, and other physical injuries and stresses commonly cause depression, even if the person was otherwise emotionally well. "More than half of all people who suffer a traumatic brain injury will become depressed in the year after the injury". In this case, he mentions a debilitating 50 mile run, a bike accident, and an operation, which very well could be the main causes rather than any job or emotional stress.