Does increasing serotonin levels in the brain really alleviate depression? or is it far too simple an assumption upon which ssri meds are based?

Depression: This is thought to be the mechanism of action of ssris.

Depression: Depression is a multifactorial disease that can be situational, episodic, or neurotransmitter based (nb). Ssris are a major treatment option for the nb type. Snris modify norepinephrine levels in the brain. There are also ssri/snri combinations. So to answer your question, flooding the brain with serotonin will not treat all depression.

SSR's made simple: Serotonin plays a dominant role but not the only role in depression. The neuroscience behind this remains a mystery and stems in a more cortical part of the brain. Few neuropsychyatric pathways of depression have been identified and interestingly some do not involve serotonin.

It seems to...: The immediate effect of ssris is to increase serotonin in nerve synapses, & since ssris seem to help depression, we conclude that increasing serotonin helps. But no one knows why. Some feel it's a downstream effect on protein synthesis, since this may explain the 2+ week delay in antidepressant action. It's far too simple to assume "low serotonin = depression." no direct evidence of this.