Earlier quoted context omitted.
Maybe your dentist just needs more business? Im half kidding but i once had a dentist that told me I had 4 cavities that needed to be fixed, but when I to a 2nd dentist, she spotted zero cavities
IIRC there were studies saying that flossing isn't as great as people suppose it is. My completely uneducated guess is that it's marginally useful for people with good teeth (shape, spacing) because debris doesn't accumulate as easily, and brushes are effective. For my case (crooked teeth, very narrow gaps) it's a great helper to get out the debris, morsels of food etc. that the brush can't get out.
Edit: I forgot to mention, when I was younger, I used to snore and breathe through my mouth a lot. I think this has also negatively affected my teeth.