Nerve fibres, the way that pain is sent through the body, aren’t present in tooth enamel. While the acid is attacking the enamel, you can’t feel a thing.
Once the acid has begun to create a cavity and attacks the dentin under the enamel, the nerve fibres begin to send out a message that something is wrong. But by then, you have tooth decay.
That’s why it’s important to have regular, preventive dental check-ups. Don’t wait until it hurts!