Depression is sadness than can last a few days or weeks, or a long time. It can also manifest as a loss of enjoyment in life, even in the activities that once gave a person great joy and sense of self. It is characterized by feelings of sadness, despair, and discouragement. It often follows a personal loss or an injury, like TBI. When someone's life has changed drastically, it's normal to feel depressed. It is not a sign of weakness, nor does it represent a moral failure. It is definitely not something to be ashamed of.
Depression can result from the chemical changes in the brain itself. It can also be a symptom of post-traumatic stress disorder. If the depression becomes extreme and affects the way you live your life, it may become pathological. Seeking help early is crucial.