A healthy smile!
Dental treatment needs to be done on two levels: esthetically and functionally. What that means is that the loss, the abrasion of teeth can lead to a number of disorders that can cause chewing, speaking problems, teeth drifting our of their position, alternate your bite forces that leads to stress in the mandibular joint, face muscles, in severe cases even pain.
So by making dentures, implants, crowns, bridges or even removable dentures, the clinician needs to regain the "neuro-anatomical" function, along with the esthetic. That is why I believe a smile needs to be healthy.