Why are so many physical symptoms in the definition of depression?

If you take a close look at the diagnostic criteria in Appendix B, you will see that physical problems such as insomnia, fatigue, low energy, physical slowing, and change in appetite are an essential part of major depression. No one can be given the diagnosis of unipolar major depression unless he or she experiences several important physical symptoms. This is one of the keys to understanding that unipolar major depression is a real medical disease like a heart attack, high blood pressure, diabetes, or hepatitis. The physical symptoms illustrate that there are powerful physical and neurological forces at work in unipolar major depression.