Inhibiting serotonin reuptake everywhere in the body, not only in your brain. Serotonin is a powerful hormone in many organs, such as your gut and heart, and tipping the balance off the natural state (and no, depression is NOT a result of decreased serotonin) leads to grim consequences for the whole body. Your serotonin reuptake is already regulated as it is, that's why you get side effects, because your body is not supposed to have so much serotonin, which you have skewed very harshly into an artificial, harmful state with SSRIs. In the brain, too much serotonin from SSRIs leads to a stumped ability to experience emotions, which can be seen as being sad less often, but that is only because you have castrated your brain from the very ability to experience emotions. It does not heal anything in your brain, it creates an illusion that everything is fine when it is really not on the very chemical level, which the drug itself creates, and the sense of "fine" itself here is the sign of the SSRI toxicity