Cavities usually don't hurt.
What?!
Anyone who has EVER had a toothache is likely to think . . . BS -- worst pain I ever had. And that could well be true. But here's the thing -- and we'll say it again -- cavities usually don't hurt. When they do, we are typically talking about the end-game. That tooth is in grave danger.
In fact, in the early stages of decay teeth hardly ever hurt.
There's a good reason for that. The outer layer of the tooth -- the enamel -- is about 97% mineral. The rest is basically water. So there is nothing that CAN hurt. There are no nerves in the outer layer of the tooth. You know, nature is just not that stupid. It wouldn't make sense to put nerve endings into something that is constantly exposed to biting and consuming hot and cold things.
Beneath the enamel you have dentin. Now, that has some sensation and, if it weren't numbed during a dental procedure, you would likely feel pain if your dentin were to be drilled. But not your enamel. The funny thing is that even cavities that do eventually reach the dentin often don't hurt. It takes getting close to the pulp -- the very center of the tooth that contains the nerves and blood vessels -- before you get pain. But then again, sometimes you don't. Sometimes the tooth just dies. Teeth can be weird that way.
The best thing to do is get your teeth checked regularly. Your doctor can tell if the cavitiy is deep enough to require filling or if it still has the capacity for repair. Whatever you do . . . don't wait for the pain. By the time you feel it, it's usually pretty bad.