I have been having toothaches on this one tooth for years. can someone explain the cause?

Toothache: You really need to see a dentist for an exam including x-rays to evaluate the situation.

Many possibilities: Previous "nerve" damage, perhaps from deep filling, partial loss of vitality, fracture, traumatic occlusion, hyper function, history of traumatic incident, gum disease, abscess, cyst, neoplasm, and many more. See your Dentist to check vitality of tooth and integrity of supporting tissues. Referral to an Endodontist may be in order.

That's a long time: Have you considered going to see a dentist? There are so many causes, from fractured teeth to non-vital nerves. This problem is best diagnosed with an exam and radiograph.

Dental Treatment: Regardless the cause of your toothache, dental treatment is the only option for your dental and periodontal care. Take NSAIDs or Tylenol (acetaminophen) OTC and see your dentist ASAP for evaluation, x-rays and proper treatment.