At the time a had a small tooth ache so I was planning to visit a dentist to check it. I was living in Switzerland, where dentist costs are very high, but I was about to go on holiday to Tenerife, where prices are much lower. So I checked on the Internet and chose the dentist with the best reviews near my holiday resort. When I was there and had explained my problem, the dentist said he'd like to do a full check of my teeth. He did that with a little camera so I could see my teeth on a screen as well. Next he went past all my fillings. I had quite a few of them, and for every one of them his comment was the same: 'decay'. He said they were old, or didn't fit properly, so around the edges decay had started. And indeed the image on the screen showed some brown edges. In the end he offered to replace all of them with some modern alternative. The treatment would cost a couple of thousand euro and would take my entire holiday week.
I didn't entirely trust him so I said I needed some time to think about it. Back in Switzerland I went to my usual dentist. I didn't tell him about the Tenerife episode but just explained about my complaint and asked him to check. He concluded that there was not really any problem, it might be my gum was infected and this gave me some pain. Then I told him the diagnose of his colleague in Tenerife. I expected that he would be shocked but he wasn't: he asked how much the guy was charging and just said it wasn't a bad price.
I then told the story to a friend whose both parents were dentists. He wasn't surprised either and he explained like this: he said dentists were like car mechanics. When a car mechanic is checking your engine and some parts were a bit worn out, some would say they should be replaced, but others would say they are still working so let's only replace them when they break.
It's 5 years later and all those 'decayed' fillings haven't given me any problem so far. But maybe I was just lucky? I'm completely confused now.