It unfortunately depends on more than just your dental care. My family has rubbish teeth - both my mum and sister have some sort of saliva issue that means they tend to accumulate plaque-forming bacteria really easily, so they have to clean really well and even so they've both had a few fillings. My Dad's teeth just crumble, they're awful, and I don't think it's because he doesn't take care of them, it's just the way they are.
I'm reasonably lucky and have only had one proper filling. When I had milk teeth though, some of them had weak enamel because I was given some kind of antibiotics as a baby (tetracycline? not sure) which can have that effect. So they had to be painted with extra enamel which was vile, but not quite the same as fillings! Fortunately the antibiotics don't seem to have affected my adult teeth apart from a tiny stain on one.