Nothing lasts forever. I have had LASIK (Laser-Assisted in situ Keratomileusis) surgery and can now see 20/20. I know however, that at some point the lasik will need to be re-done.
Get a knee, hip, or any other part of your body replaced and the surgeon will likely tell you how long you probably have till the prosthetic piece, or your body around the piece, goes bad.
Fillings in teeth are no exception. They have a certain life span and then, the need to be replaced.
Here is an Old Metal Filling. 
Here is the New White Filling. 
This will not last forever either, but it will function far better then the old cracking metal that used to be there.
