Sadness—a core emotion—is a signal that you have experienced a loss, actual or imagined. Sadness provides energy on a temporary basis to help discharge painful feelings, grieve losses, heal past woundedness, recover, and return to a state of feeling better.
—Arlene R. Taylor

Without sadness you may fail to recover from your losses and/or grieve successfully.
—Arlene R. Taylor

Prolonged or unmanaged sadness can suppress immune system function leading to illness; may decrease levels of serotonin that helps with mood; and can lead to despair, immobility, and apathy—or to depression and even suicidal thinking.
—Arlene R. Taylor