A tooth can hurt for a couple of reasons. If you have a large filling, it leaves less tooth. Does that make sense? The smaller the filling, the more tooth left. The larger the filling, less tooth left. The thinner the tooth walls, the more fragile they can get, and can develop fractures or cracks which can cause sensitivity to pressure or hot or cold. Also, if decay gets too close to the nerve chamber, the center of the tooth, you can get the same sensitivity. You can get the decay removed and a filling in place, but the sensitivity may remain because of its close proximity to the nerve chamber. If the sensitivity persists, that's when your dentist will recommend a root canal to remove the nerves transmitting pain. The last reason a person may need a root canal is because of infection, or an abcess from a dead or dying tooth. One of the reasons why a person needs to go to the dentist regularly is to have teeth checked for any new decay, as well as how any fillings or other restorations are holding up. Older fillings tend to pull away from the tooth, allowing bacteria to get underneath and start a cavity under the filling. Sometimes your dentist can see this visually, sometimes it is with the aid of x-rays, or bitewings. There are really good dentists that make the visit easier. I was an assistant, but still hate the idea of my mouth being worked on. The office I worked at delt with a lot of high fear adults, and we took steps to make each patient feel at ease. We had a lot of compliments. Most adults avoid going to the dentist because of some bad experience when they were kids. Take the time to find a good dentist, it will make all the difference in the world.