By the time a tooth actually begins to cause pain, the problem is usually quite severe, and has been there for a long time. Most types of dental disease do not cause any pain at all, and are not easily visible, which can give you a false sense of security.
For example, if you have a cavity (decay / caries) form in a tooth, it will usually take months or years for it to actually cause you any pain. This is why it is crucial to maintain regular visits to the dentist so that any problems can be intercepted early on. Nipping a problem in the bud early is cheaper and less painful than waiting for it to start hurting.