Looking After Your Teeth Doesnt Always Pay?

Why is it that some people have really amazing teeth with not a single filling in there head? Usually there the ones who eat loads of sweets and brush once a day an never floss.
My ex has a sweet tooth his 28 year’s old and not a single filling he doesnt look after his teeth as I use to basically live with him an see how he treated his teeth. I am 20 year’s old and had my first filling just over a year ago, I no have 1 root canal, and 4 fillings I brush my teeth twice a day and use colgate sensitive gentle care mouthwash, I am a little scared of flossing as I feel my fillings will get pulled out.
It’s annoying I asked the dentist why I am having such bad teeth an her answer was just brush and floss, Im like but I do. Why do some people have great teeth an others dont?
I’ve been dentist 3 times since February, it’s really starting to get me down an it’s very expensive.