Pretty much every person today by the age of 25 year has several teeth with fillings. I actually knew few people who already lost a tooth by that time (i.e. depulped tooth, refused to put a crown, tooth cracked).
It’s astonishing that galley slaves and other malnutritioned people actually had no problem with teeth. Yeah, they did not live long, but what I want to emphasize is that there is nothing forcing you to have cavities in teeth. It is also well known that dental caries epidemic started approximately the same time as sedentary agriculture.

I will not dwell on the subject too much — you can find or already saw the information everywhere else. So the bacteries and acids from the food wash out minerals from teeth and saliva replentishes the minerals. If you leech more minerals than replintish on a regular basis — you get a carier.
Lesser known but still known fact is that the carier does not lead to pulpitis right away, because inside the tooth there is a similar mechanism of “leech-replentish” going on. It goes like this:

1. Caries 2. Deep Caries 3. Pulp recedes

Caries goes deepper and deeper with a speed depending on your Coca cola daily consumption, and when it starts impacting the pulp — pulp recedes leaving behind a calcified material. If the rot hapens faster than the pulp recession rate — you get a pulpitis. Of course, the recession cannot go forever, but the core concept here is that teeth have high durability by nature.

Classical mechanism of fast tooth decay is: there is a food getting stuck in some place for a long time, bacteries hang out a lot in this place and eat your tooth out. Clean up the bacteria or clean up the food remains — and you got yourself a healthy tooth. Modern people invented new routes of dental tortures — it’s sugar and particulary acidic soft drinks, the latter can give you a “meth mouth” in a relatively short period of time if consumed in sufficient quantities.

Today there is a movement towards conservative treatment of caries, however, most of dentists simply don’t give a shit about it. It’s not that they are evil, it’s that the basis of their profession is to not give a shit — otherwise they would teach you how to have healthy teeth and never visit a dentist.
Orthodoxal dentist would remove as much material as he can and fill the cavity. Modern conservative approaches try to avoid destroying deep tissues and instead let the tooth heal, by enclosing the cavity and stopping the caries completely:
https://onlinelibrary.wiley.com/doi/10.1111/j.1600-0722.2010.00731.x — Treatment of deep caries lesions in adults: randomized clinical trials comparing stepwise vs. direct complete excavation, and direct pulp capping vs. partial pulpotomy
https://www.sciencedirect.com/science/article/abs/pii/S0002817722004652 — Deep caries removal strategies: Findings from The National Dental Practice-Based Research Network

Morale of the day: don’t eat sugar-reach crap, clean your teeth carefully, and remember — the best dentist is the one that you don’t have to visit.