system
Given the following post, respond as if you're commenting on Reddit:
human
[ASOIAF] Why doesn't Thoros of Myr and his Lord of Light bring back every member of the Brotherhood without Banners that dies?
gpt
Thoros isn't a real red priest, well he wasn't a real believer in the Lord of Light even after he was sent off to them as the youngest child of 8 siblings. he cared too much for fighting and drinking to ever take the religious part seriously, which is why he was sent to Kings Landing - King Robert shared similar interests and kept Thoros at court as a drinking companion who competed in tourneys  regularly to the amusement of the King. He never swayed the King from the faith of the seven, but he never really tried because he didn't believe it himself. 

when Beric fell he didn't know what to do, so he performed the rituals he had been taught as a red priest and somehow Beric woke. perhaps it was because magic re-entered the world when the dragons were born, but after that many of the tasks he used to perform just because he was a priest and was doing what he was taught actually worked - like seeing visions in the flames and actually lighting swords on fire using prayers. 

he performed the last kiss many times to many fallen companions, but it only ever brought back Beric and only after they were sent out to capture Clegane.

he even refuses to use the last kiss on the body of Catelyn Stark when they find her rotting in a river. Beric passed the flame to her. he was broken after being brought back so many times, he didn't remember his own lands, his castle, his people, his fiance, he even forgot who knighted him. Thoros did him no favors by bringing him back - he was carrying out a ritual he was taught as a child - it never brought people back before and after he was worried who would come back, Beric is quoted saying "fire consumes. it consumes and when it is done nothing is left. nothing" and thats the truth and why Thoros doesn't bring back everyone. Beric is a rare case in that he was one man in a thousand who still fought for justice when he was brought back, he didn't know why or for who, but he fought for it. we see the ill effects when he passes on the gift to Lady Stoneheart who cares not for justice and just wants to see the Freys hanged - even if they had no hand in the Red Wedding.

Thoros doesn't really have control of his power, according to him his god uses him as a tool, but to us maesters it seems more likely he never really had any power and magic somehow finds its way into our lives and disrupts them.