This may sound a bit surprising to most people, but the large majority of cavities are completely painless. This is because the outer enamel has no nerves. It is only when the cavity enters the underlying dentin that the cavity may begin to feel sensitive. The most common symptoms are an increased sensation to cold, sweet foods or beverages. A cavity is often responsible for a tooth that breaks. The cavity weakens the tooth, especially when it forms under a filling or a tooth cusp, and can easily cause a fracture when biting down.
Patients are sometimes taken off guard when they learn that they have a few cavities but they don’t have any symptoms. It is far better to treat a small cavity than to wait until they have symptoms, such as pain. By the time there are symptoms, the cavity may have spread to infect the dental pulp, necessitating a root canal procedure or an extraction to eliminate the infection. Always remember that most dental problems are insidious — that is, they sneak up on you. Regular dental exams, at least twice a year, will greatly reduce the likelihood that a dental cavity will go undetected and spread, causing pain and infecting the dental pulp.
Source: http://www.dentistry.com/