Warning: potential spoiler for the viewers of the TV show Game of Thrones : as of season 5, one of the concerned characters has not yet been introduced, and may or may not appear in future seasons of the show
Thoros of Myr, being a red priest of R'hllor, can bring dead people back to life. In a A Feast for Crows, he explains
the return of Lady Catelyn Stark as Lady Stoneheart
to Brienne of Tarth:
"When we found her by the river she was three days dead. Harwin begged me to give her the kiss of life, but it had been too long. I would not do it, so Lord Beric put his lips to hers instead, and the flame of life passed from him to her. And... she rose. May the Lord of Light protect us. She rose."
A Feast for Crows - Chapter 42 (Brienne VIII)
He explains his refusal with "it had been too long, I would not do it". But too long for what? I can't interpret whether the "May the Lord of Light protect us" part is the surprise that it actually worked, or the fear of what she became.
Thanks to his experience with Beric Dondarrion, he probably suspect what kind of damage ressurection after a long death can do, so I can see him not wanting to create what could be some kind of soulless monster.
On the other hand, he doesn't seem to have prevented Beric from trying. And doesn't oppose her in becoming the new leader nor leave the Brotherhood after that.
So, do we know what was his deep motive to refuse?