Feeling sad all the time indicates that a person is suffering from clinical depression, according to WebMD. This includes feeling hopeless, worthless, pessimistic and guilty.
Seasonal Affective Disorder (SAD) is another cause of feeling sad all the time, according to Mayo Clinic. This condition is depression that usually arises during the winter.
Individuals who notice their sadness is affecting their everyday lives should seek help. This includes feeling unable to work, being unable to maintain relationships and having suicidal thoughts. Teenagers who feel sad consistently should seek assistance after two weeks, WebMD states. In addition to mental symptoms, depression can present physical symptoms, such as not eating, not sleeping and stomach pains.