Just as headaches are not caused by a lack of aspirin, the efficacy of serotonergic drugs are not a proof that depression is caused by a lack of serotonin.

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"You've all heard the analogy that depression is like diabetes, a "chemical imbalance" in the brain. The chemical most often mentioned as imbalanced is a neurotransmitter called serotonin, which is hypothesized to be low in depresed people. Fortunately, like the diabetic, the depressed person can take a drug like Prozac that corrects this imbalance and raises serotonin to normal levels.

The idea that low serotonin causes depression has been repeated like a mantra by public health authorities ... . The serotonin theory of depression is a good illustration that if you repeat something enough times, people will come to believe it.

The widespread use of serotonergic drugs for treating depression and evidence for their efficacy has also greatly boosted the serotonin theory of depresion (even in the face of evidence that the beneficial drug effects are relatively modest).

But just as headaches are not caused by a lack of aspirin, the efficacy of serotonergic drugs are not a proof that depression is caused by a lack of serotonin.
...
Now recent data present even more serious problems for the low serotonin theory of depression. New findings that directly contradict the core of the theory: A paper in the flagship journal, Archives of General Psychiatry found evidence of increased serotonin activity in depressed persons. Furthermore, growing evidence suggests that it is an error to even talk about the brain having a single serotonin level.  Based on work with rats and mice, neuroscientists are increasingly moving to the view that there are different populations of serotonin neurons that are each independently regulated.

As a scientific venture, the theory that low serotonin causes depression appears to be on the verge of collapse. This is as it should be; the nature of science is ultimately to be self-correcting. Ideas must yield before evidence."