WTF TEETH?!

Dentist appointment today and it wasn't good. Two of my 3 fillings have apparently become unsound and have to be replaced. In fact, on one of those teeth, part of the filling has actually come out and I've now got the start of a new cavity in the exposed area. Argh. I go to the dentist every 6 months for check-up and cleaning, so I feel like the dentist should have at least noticed the worse of the two getting loose. And why do my fillings seem to need replacing every couple years? Does this happen to anyone else? My dentist told me it's a common occurrence, yet I know no one else it's ever happened to. So not fair.