science

No. We don't. While obviously, we know it inhibits serotonin reuptake, we do not know why that SSRI action works. They were developed based on the monoamine hypothesis - basically "too little neurotransmitter activity, better beef it up". And so it does! In a matter of days. Any actual antidepressive effect notoriously can take weeks or months, so that obviously isn't the entire mechanism.