In the brain without an ssri the body breaks down used serotonin with an enzyme
and then makes more. With an ssri
serotonin is directed to targeted receptor sites, which differ depending on the medication, and breakdown is prevented so the serotonin washes around longer in the targeted sites. Other sites, in turn, shut off because the brain doesn't see them as necessary anymore. You have the same amount of serotonin, it's just hanging around longer in the targeted receptors.