That seems like a good question. However the desired results is to regulate the amount of serotonin. It is not simply adding serotonin. It is similar to a diabetic who does not have the right amount of insulin. Simply controlling what the patient eats may be enough for those not too bad, but many need to regulate how much insulin they need. For those who are not that depressed psychotherapy may be enough; however, for those with severe depression there's a need to regulate by medicine. The ssri is a selective serotonin reuptake inhibitor. It thus selectively inhibits the uptake of serotonin, thus increasing the amount you have. The problem with depression caused by chemical imbalance is that your body produces too much and then tries to reduce the amount, but then ends up with too little. By increasing the amount of serotonin, we have broken that cycle. So it is similar to what happens in a diabetic controlling insulin. That is why as a diabetic cannot stop their insulin, so we should not stop our medication without a doctor's orders. I hope this helps with understanding how it all works.