Answer: The most common reason for this is because the bacteria that destroyed the tooth went too close or into the nerve of the tooth. Sometimes a special dressing can be placed under a filling to help prevent a root canal but sometimes that is not always the case. The bacteria being so close to the nerve inside the tooth can irritate it and cause it to die – making it very sensitive to hot or cold temperatures. Left untreated a dying nerve will eventually become infected and will result in the dental pain that wakes/keeps you up at night. Do note that if it hurts to chew AND you have hot/cold sensitivity with a newly placed fillings, the most common reason is the filling was placed too high and a simple adjustment will fix that problem without a root canal being needed!