Silver fillings made of amalgam don't last forever. With time they may show signs of cracking, leaking or chipping. The tooth itself may weaken also, depending on the size of the filling. This signifies the filling is at the end of its lifespan and we tend to recommend replacing the filling at this time. A more modern material to use is a composite resin which is tooth coloured and more aesthetically pleasing than amalgam.
Additionally, amalgam fillings do contain trace amounts of mercury which has seen a rise in patients electing to have their fillings replaced sooner rather than later. This is a personal preference and an elective procedure, as old amalgam fillings are considered safe.