i had this discussion with my psych not so long ago. there is a lot of evidence that shows that low seratonin levels can contribute to depression but don't actually cause it. we just find that people respond quite well to increased levels of seratonin.

a lot of people suffer depression and it doesn't react to meds because it is not a chemical issue but a situational depression which can only be solved with time and changes in circumstances.

which is why meds alone are not the answer