You might want to take a look at this really interesting article, "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. The little blue pills cheer us up because they give the brain what it has been missing.There's only one problem with this theory of depression: it's almost certainly wrong, or at the very least woefully incomplete. Experiments have since shown that lowering people's serotonin levels does not make them depressed, nor does it does not make them depressed, nor does it worsen their symptoms if they are already depressed.
(HT: evangelical outpost)