The LA Times has an interesting article on evolutionary theories of depression that also discusses how these might lead to new and improved treatments for the condition.
The fact that mental illness is both widespread and disabling is a puzzle in evolutionary terms, if you believe that a vulnerability to psychological disorder is strongly inherited.
Indeed, the evidence suggests that there is a significant inherited component in mental illness, although the extent of this influence is debated.
If this is the case, the question arises ‘why do we still have mental illness if inheriting the risk for it makes you much less likely to reproduce?’. Surely it should have been ‘bred out’ of the population?
Some use this as an argument to suggest that the role of genetics in mental illness has been overstated, and that the majority of risk arises from environmental factors, particularly those that cause stress and trauma.
Others suggest that the same inherited attributes that increase risk for mental illness can be beneficial when they don’t result in serious impairment.
For example, research has suggested that people who are at high risk for schizophrenia, or have slight or fleeting psychosis-like thoughts, are more likely to be creative or original thinkers [pdf].
More recently, it was reported that a gene called DARPP-32 increases risk for schizophrenia as well as being linked to the more efficient use of a key brain circuit in the frontal lobe.
This might explain why genes that increase these tendencies are still in the gene pool, and only when too many of these traits are inherited is the person very likely to suffer ill-effects when confronted by severe life stresses.
A similar theory was put forward by the late Dr David Horrobin is his book The Madness of Adam and Eve: How Schizophrenia Shaped Humanity (ISBN 0593046498).
As an aside, Horrobin was famously the subject of a controversy after a critical obituary was published in the British Medical Journal, leading to an angry reaction and the journal publishing an apology.
The LA Times article is a great overview of evolutionary theories of depression that might help answer questions about why someone might inherit a tendency to be depressed.
If this tendency is understood as an exaggerated form of something that might be beneficial in small doses, it may give clues to new treatments, and the article looks at what treatments researchers are considering with this in mind.
Link to LA Times article ‘The mind, as it evolves’.
Mind Hacks 
Regarding the evolutionary origins of mental illness (in particular Schizophrenia), or what is sometimes called the darwinian paradox (e.g. Why deleterious traits persist when those traits prevent biological fitness- the quantity of offspring organisms leave-)
T. Crow is suggesting that schizophrenia is at the basis our unique capacity to speak and language. But he shows skepticism with some evolutionary explanations because some of the advantages that they posit are not at the level or in the same dimension of the disadvantages, say, we cannot expect that a given disorder centered around a cluster of disfunctions could improve certain functions center around another cluster.
When talking on depression some evolutionary explanations suggest an anlogy with pain as an interoceptive or homeostatic signal, but in the case of depression, as a signal that some behaviours of the individual is compromissing its fittness.
In this point i´m skeptic as well, because women that have been mothers recently but suffer post-partum depression, why they have depression interpreted in this sense; having a child increase their fittness don´t you think?