There's a very interesting recent article on boston.com about how our understanding of depression continues to shift as we learn more.
How Prozac sent the science of depression in the wrong direction
Like many other antidepressants, Prozac increases the brain's supply of serotonin, a neurotransmitter. The drug's effectiveness inspired an elegant theory, known as the chemical hypothesis: Sadness is simply a lack of chemical happiness.
There's only one problem with this theory of depression: it's almost certainly wrong, or at the very least woefully incomplete.
In recent years, scientists have developed a novel theory of what falters in the depressed brain. Instead of seeing the disease as the result of a chemical imbalance, these researchers argue that the brain's cells are shrinking and dying.
the drug works because it helps heal our neurons
In this sense, Prozac is simply a bottled version of other activities that have a similar effect, such as physical exercise. They aren't happy pills, but healing pills.
These discoveries are causing scientists to fundamentally reimagine depression.