Why does the same tooth keep getting infected after a root canal and 2 re-treatments? even my dentist was clueless!

Retreatment failure: the anatomy of the canals in a tooth are very complex. In spite if dentistry's best efforts it is possible that there are undiscovered canals or areas of retained dead tissue. These can be from accessory canals that can become blocked during treatment. Its also possible that the tooth is fractured and is therefore unrestorable. Unfortunately the only definitive treatment is extraction.

Endodontist: Have an endodontic specialist take a look. The tooth may have a problem that your dentist missed. Even the best care can still result in failure of the treatment.

Not uncommon: The endodontic re-treatment success is about 85%. This means that about 15% of root canals re-treatment will continue to fail and will require either microsurgery or extraction and implant placement. Ask your dentist for an alternative treatment.

Accessory canals: Sounds like evaluation by root canal specialist, an Endodontist, who has the training, experience, Endodontics microscope, and COne Beam Cat Scan to evaluate for accessory canals, fracture, leakage, and/or perforation.