My first impulse was to say that Houndie had lost everything else, and that the only thing left that meant anything to him was revenge... but they didn't really work it that way.
Hound never had a life, no friends, no lovers, no relationships, no money, no lands, only known family was a mortal enemy. I suppose it could have been that once he'd fought in the war to end all wars he realized he had nothing to look forward to and nothing to live for but his old grudge... but because this season is so rushed and badly written they didn't actually write him that way.
Refresh my memory ... what was Sandor’s arc with Ian McShane and Beric all about?
Because it seemed like the “killing my brother is no longer a grudge ... it’s now my life’s mission” thing came out of nowhere.
I actually though Sandor would be revealed as an agent of the Lord of Light and Gregor’s zombie to be part of the Night King’s agenda, but nope.
Well a lot of stuff has come out of nowhere this season, they're trying to cram two seasons' worth of stuff into six episodes and they're leaving out everyone's motivations. I mean, Hound has been at loose ends since Brienne whupped his ass and he nearly died, he joined the Brotherhood and then he went to fight the Others with them and I suppose he was at another loose end when the NK was beaten... but he could have started a new life in the North or somewhere else. Sansa would have been happy to give him land or get him a job as the captain of some Lord's castle, or he could join the Wildlings or see if Yara would like a fighting man, whatever.
Instead, he's going on a suicide mission for no reason that anyone can see.