Jonathan Rottenberg suggests that a “keen awareness of what has gone wrong and what can go wrong again can help a person avoid similar stressors in the future”:
In humans the value of low mood is put to the fullest test when people face serious situations in which current problems need to be carefully assessed. We might think of the groom who is left at the altar, the loyal employee who is suddenly fired from his job, the death of a child. If we had to find a unifying function for low mood across these diverse situations, it would be that it functions like a cocoon, a place to pause and analyze what has gone wrong. In this mode, we will stop what we are doing, assess the situation, draw in others, and, if necessary, change course.
A variety of experimental data have shown that low mood confers benefits to thinking and decision making. That lends credence to the idea that mood is part of a conservative behavioral guidance system that impels us toward actions that have been successful in the past—meaning, actions that helped our ancestors to reproduce and spread their genes. One way to appreciate why these states have enduring value is to ponder what might happen if we had no capacity for them. Just as animals with no capacity for anxiety were long ago gobbled up by predators, without a capacity for sadness, we and other animals would likely commit rash acts and repeat costly mistakes. Physical pain teaches a child to avoid hot burners; psychic pain teaches us to navigate life’s rocky shoals with due caution.