It makes sense now that we've seen him spend five episodes becoming steadily more and more disillusioned with the maesters […] He had a dream of maesterhood that's proven to be very different from the reality he's been experiencing, and now he thinks there are better ways he can serve Westeros, so he's off to do them.

Also, once the gang is assembled at Eastwatch, why doesn't anyone suggest updating the plan? They don't need to try capturing a wight when they can go south, stab Beric, have Thoros reanimate him, and then explain "there are less friendly corpses up North btw, maybe let's kill them instead of each other."

I think my favourite obvious line at the end of a Times op ed is "Barack Obama is the president of the United States."