Depression is a type of illness, not a personal sign of weakness. Possible symptoms include feelings of worthlessness, irritability, and a loss of interest or pleasure in hobbies and activities that were once enjoyed. Although research scientists and doctors do not know the exact cause or causes of depression, they do believe that the onset of depression is frequently caused by a combination of genetic, psychological, and environmental factors. In order to diagnose depression, a healthcare provider will begin by asking a number of questions and will then perform a physical exam, looking for signs of conditions known to cause depression.