I strongly think that for every depression there is a reason. Or multiple. Doesn't matter really. If you find it and deal with it, it's over. Medication is only treating the symptoms, but not the cause.
You could not be more wrong. Depression isn't "man, I'm just really sad!" it's an actual chemical imbalance in the brain. Medications correct that chemical imbalance and starting on an anti-depressant takes WEEKS to start working and many times many trials and errors before finding the right medication and dose that will work.
Anti-depressants DO treat the cause! Because the cause is a chemical malfunction in your brain.