Variation in Mood:

Randolph M. Nesse:

"In the face of threats and losses, anxiety and sadness are useful, but happy relaxation is worse than useless. When opportunities emerge, desire and enthusiasm are useful, but worry and sadness are harmful. The advantage goes not to individuals who are constantly anxious, sad, or joyful, but to those who experience anxiety when loss is threatened, sadness after a loss, and enthusiasm and joy in the face of opportunity and success."