What do you mean 'I need a filling'? I brush every day!

You brush and floss every day. You have a healthy diet. You don't like chocolate. How can you possibly need a filling? "UNPOSSIBLE!"

Every day dentists come across this situation. Good oral hygiene, but a cavity developing. There are several possible explanations. It could be that the cavity got started years earlier and has only now got large enough to identify. Another explanation is that, due to the three dimensional shape of the biting surface, it isn't possible to brush right down to the bottom of the 'valleys'. Another possibility is a natural weak spot in the enamel.

How to prevent this? Travel back in time and advise your teenage self to brush and floss, use a fluoride mouthwash and change to Coke Zero. Whilst you're there send your teenage self to get their (your?) teeth fissure sealed.

"So it's too late for me?". Well, it is too late to prevent something that has already happened, but it isn't too late to prevent future problems. The recipe is the same, brushing (2, or better 3, times a day), flossing, fluoride mouthwash, reducing sugar and maybe getting your teeth fissure sealed.