I never said that depression is not a chemical disorder, merely that it is chemically instantiated rather than caused. Considering that even psychiatrists can't actually explain with certainty why pharmaceutical treatments for depression are effective (as treatments of symptoms -- not cures), asking me to explain why they work is a bit misdirected. I would say the same theory most psychiatrists have for why it works is what I'd believe too -- reduced serotonin reuptake making it easier for action potentials along serotonegenic pathways, if it's an SSRI.

No one is disputing the chemical route to symptom management and how massive and life changing an improvement it can be. But just as a painkiller working for pain management does not mean an injury was biologically or genetically caused, neither does the efficacy of an antidepressant indicate anything one way or another about the roots of a depression. Could be genetic, could be circumstantial, could be biopsychosocial. Either way, I'm glad you've found a treatment that really works for you.