"Positive emotions, such as love and joy, seem useful while negative emotions, such as anger and anxiety, seem maladaptive. This is an illusion, the same "clinician's illusion" that makes it hard to see the utility of bodily defensive responses such as fever and pain. The high costs of negative emotions and body defenses demonstrate not that they are useless, but the opposite: their high costs confirm that they offer substantial benefits in certain situations. They will sometimes be expressed in situations where they are maladaptive, and the systems that regulate defenses make mistakes (Nesse, 200lb; Nesse, 2005), but overall they give an advantage.

Although it is harder to see the utility of negative emotions, most people readily recognize the utility of anxiety. In the face of a potential danger, protective responses prevent damage or loss. Sadness, however, typically occurs after a loss. The horse is already out of the bam, as it were, so how can any kind of response be helpful now? However, an evolutionary view highlights different questions: Is the loss of a valuable resource an event of adaptive significance? To put it more specifically, are there things an organism can do after a loss that might increase its fitness? There are many, including attempting to undo the loss, trying to prevent future losses and warning others."

An Evolutionary Framework for Understanding Grief, Randolph M. Nesse, 2005

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"Sadness differs from low mood and depression. A discrete loss that does not block long-term goals will arouse sadness that soon fades. If, however, the loss makes it impossible to reach important goals, continued striving toward an unreachable goal gives rise to low mood, that is, mild depression. Klinger describes how low mood can be useful in disengaging effort from unachievable goals and how persistence in pursuit of such goals could escalate ordinary low mood into full-fledged depression (Klinger, 1975). This paradigm has now developed so it is widely recognized that mood is influenced by events that indicate a change in expected rate of progress towards crucial goals (Carver&: Scheier, 1990, 1998) although recognition of the relevance of these discoveries for psychiatry is still nascent (Nesse, 2000a)."