10 months ago, Redditor joshmarshall07 posted his own theory about why Beric has been brought back from the dead so many times: Apparently in the books, Beric sacrifices himself to bring Catelyn Stark back from the dead (she becomes Lady Stoneheart, which we don't see happen in the show), and it's theorized that maybe he'll bring back Jon or Daenerys if need be:

"Guess why the Lord of the Light keeps bringing Beric Dondarrion back. In the books Beric gives his own life to bring Catelyn Stark back to life. I think its going to be the same in this case of series too. Maybe Jon will die again or Daenerys or someone important that the Lord wants for the war. Since Thoros is dead in the series, Beric will give his own life to resurrect the person as of the books."