Rian Johnson had (at least) one request for J.J. Abrams about The Force Awakens‘ ending – to have Artoo go with Rey instead of BB-8, so the classic droid could be reunited with Luke Skywalker. Luke, naturally, is central to The Last Jedi:
“Figuring out where his head was at was the very first thing I had to do when writing the movie. I had to crack this. And it had to be something for me that first and foremost made sense. Why did Luke Skywalker go off to this island?” Johnson says. “That was the starting point, and that’s what the entire movie explores.”
So Luke’s self-exile – and his reasoning – is a key factor in the film.