To add to this, SSRI onset efficacy corresponds with the downregulation of serotonin receptors in the brain. "Low serotonin" isn't what's being treated or changed, but the number of receptors and their activation rates. SSRIs can reduce emotional lability, which makes sense when you figure that downregulation and increased activation of serotonin receptors is a relatively stable system compared to an unmedicated brain with many receptors and widely fluxuating activation rates and lowered activation periods as serotonin gets recycled quickly without its transporter being inhibited.