Why would a crown that's had 2 root canals 15 years ago be hurting now?

A tooth that has had two root canals and still causing you pain is not a good sign.  I would recommend that you schedule an appointment with a dentist soon.  There is a decent chance that this tooth may need to be extracted and have a dental implant placed.  Best of luck.  

Root canals are very complex systems.  The tools that we have to clean the systems out are fairly simple.  These tools are very effective over 90% of the time but sometimes there are parts of the root canal system that are difficult to access and therefore harbor tissue that can decompose over time and/or bacteria that can cause problems at a later date.  Another thing that can happen to root canal treated teeth is that the root can crack or fracture.  This can lead to the type of symptoms that you are experiencing.Please go see your dentist to assess the condition of your tooth and see how likely it is to be saved.  I had a tooth in my own mouth that fractured many years ago, was root canal treated, and kept breaking down and being repaired over those years.  A few months ago I determined that it could no longer be saved, and I had it removed and an implant was placed to replace it.Good luck ...