Just my 2 cent's (short version):
"In summary, we hypothesize that depression is a stress response mechanism: (1) that is triggered by analytically difficult problems that influence important fitness-related goals; (2) that coordinates changes in body systems to promote sustained analysis of the triggering problem, otherwise known as depressive rumination; (3) that helps people generate and evaluate potential solutions to the triggering problem; and (4) that makes tradeoffs with other goals in order to promote analysis of the triggering problem, including reduced accuracy on laboratory tasks. Collectively, we refer to this suite of claims as the analytical rumination (AR) hypothesis...
...In short, CBT, EBA and IPT are effective psychotherapies for depression, and a common feature is that they attempt to identify and help solve problems that depressed patients face. Unlike medications, they have enduring effects even after treatment has ended. Applying the principle that treating the cause of a psychological condition works better than treating a sign or symptom of it, this research suggests that depressive episodes are not usually caused by negative cognitions, but rather by problems that people have difficulty solving on their own...
...Summary
The distraction-resistant analysis of problem-related information that occurs in depressive rumination requires sustained left VLPFC activity. Based in part on evidence that behavioral depression causes sustained 5-HT release to the rodent homologue of the VLPFC and maintains neuronal activity there, we predict that brain 5-HT is high in human depression, not low. The sustained release of 5-HT to the VLPFC should promote the production of astrocytic lactate, sustain neuronal firing, and reduce apoptosis by supporting the clearance of synaptic glutamate...
...Perhaps more so than other painful emotions, people in the evolutionary past must have had to learn how to endure extended periods of depression. A complex problem, for instance, resists simple solution, and depressive pain persists despite attempts to quickly solve it. We suggest that when facing complex problems, organisms must learn to stop trying to quickly resolve their pain with simple solutions, accept a slower, analytical approach to problem-solving, and learn how to endure the pain until the problem is solved. The extended nature of depressive pain is useful. Without it, people would not be motivated to engage in the extended effort required to solve complex problems, and the pain should cease once the problem is solved. But another reason why it is important to learn to endure depressive pain is that people facing social dilemmas may anticipate further long-term pain if the best solutions require making tradeoffs (e.g., people contemplating divorce may lose children, money and home by leaving, and face continued marital problems by staying). Thus, effective decision-making will require accepting and enduring the pain that persists during analysis and the subsequent anticipated pain that arises from different courses of action...
...In ancestral environments, there were probably few ways to bypass this learning process. The persistence of depressive pain despite attempts to quickly resolve it would eventually force the organism to adopt a slow problem-solving approach and learn how to accept and endure the pain. Learning may also be facilitated through interaction with close social partners, who demonstrate or encourage the acceptance of depressive pain. Yet in modern environments, there are many ways to temporarily reduce depressive pain without solving the complex triggering problem (e.g., drugs, alcohol, distracting activities like television, etc.). Blanket statements by professionals, pharmaceutical companies, and the media that depression is a disorder may also interfere with the learning process and promote avoidant behaviors...
...The AR hypothesis proposes that avoidant behaviors: (a) bypass the process by which people learn to endure painful feelings that persist when taking a slow, analytical problem-solving approach and that arise as a consequence of considering and making tradeoffs; (b) are a maladaptive byproduct of the evolved propensity to take action that quickly reduces pain; and (c) occur in environments where the means to engage in avoidant behavior are available. It predicts that depressed people are more likely to use avoidant behaviors: (1) when they face difficult social dilemmas (because they are more likely to face painful tradeoffs that they will want to avoid); (2) in their adolescent years (because they have had less opportunity to learn how to deal with depressive feelings); (3) when they have close social partners who also engage in avoidant behaviors (because learning to be avoidant or non-avoidant is socially transmitted, in part); (4) if they feel depression and anxiety more intensely (because such people will feel greater urgency to reduce the pain quickly and be less tolerant of the learning process); and (5) if they are less intelligent (because their learning process will be slower and less efficient)."
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2734449/
