Most people who are naive about depression think that it is only about having negative emotions like sadness, anger, anxiety, and loneliness. Depressed individuals soon learn that abnormal thoughts are actually the heart of their depression and the source of most of its problems. Thoughts of worthlessness drive emotions of guilt and remorse. Nihilistic thoughts of hopelessness and death cause emotions of desperation and doom that can hijack your future plans and may even threaten your survival. Problems with thinking and decision making can also cause failures in your work, school, and personal life, producing even more negative emotions.
On the bright side, your thoughts provide you with tools to fight your unipolar major depression. Most of psychotherapy consists of changing your emotions by changing the way that you think.