The dentist must drill deeply into the softer dentin area of the tooth and drill undercuts into the healthy tooth even where there is no disease. This approach was developed in 1908 by G.V. Black. Mercury fillings also expand after being placed in the tooth. The bigger the filling the more they expand. If any moisture gets into the filling they expand rapidly.
Temperature can also cause expansion.
All this expansion within the tooth eventually results in fracture. Once broken the tooth may require a root canal or crown or extraction. Often the fracture is so severe that in spite of all efforts the tooth is lost. The majority of initial composite fillings require only minimal natural tooth removal and not only restore decayed areas but also seal up weak spots so decay will not penetrate the other surface groves. Sealed surfaces ARE protected as long as the sealant lasts.