Since Luke Skywalker didn't show up in Star Wars: The Force Awakens until the final minutes, we didn't get much of a sense of how he's changed since Return of the Jedi other than adopting the grizzled hermit look for his extended stay on Ahch-To. However, because it was revealed that Ben Solo was Kylo Ren, it was to be expected that Luke's nephew turning to the dark side would weight heavily on him. Star Wars: The Last Jedi will pull back the curtain on the events that led to Ben's fall from grace, including how Luke overestimating his nephew's potential had led to him doubting his connection to The Force. Mark Hamill explained:
[Luke] made a huge mistake in thinking that his nephew was the chosen one, so he invested everything he had in Kylo, much like Obi-Wan did with my character. And he is betrayed, with tragic consequences. Luke feels responsible for that. That's the primary obstacle he has to rejoining the world and his place in the Jedi hierarchy, you know? It's that guilt, that feeling that it's his fault, that he didn't detect the darkness in him until it was too late.
When Ben Solo became Kylo Ren, he didn't just become a servant of Supreme Leader Snoke and a leader in The First Order. He also aided the Knights of Ren in killing Luke's other apprentices, destroying the attempt to rebuild the Jedi Order with one swift stroke. Since that tragedy, Luke Skywalker has been holed up on Ahch-To, and as the first Star Wars: The Last Jedi trailer informed fans that he's now under the impression that it's time for the Jedi to "end." Thanks to EW's extensive coverage of the next Star Wars installment, we now know that Luke felt the need to exile himself because he doesn't trust his instincts with The Force. The movie won't delve too deeply into the intricate backstory Mark Hamill constructed for himself about what happened to Luke after Return of the Jedi, but we will learn more about what led to his disappearance and the guilt he suffers from on a daily basis.
Directed Rian Johnson also elaborated about why Luke hasn't shown up to aid The Resistance in battling The First Order, noting that he didn't go into exile because he's a coward, but because he needs to deal with his personal issues. According to Johnson:
The very first step in the writing of this was figuring out why he's on that island. We know that he is not a coward. He's not just hiding because he's scared. But we also know that he must know his friends are in danger. He must know the galaxy needs him. And he's sitting on this island in the middle of nowhere. There had to be an answer. It had to be something where Luke Skywalker believes he's doing the right thing -- and the process of figuring out what that is and unpacking it is the journey for Rey.
Luke Skywalker looked surprised to see Rey on Ahch-To offering his lightsaber back to him at the end of The Force Awakens, but when The Last Jedi begins, he won't give her the warmest reception. However, Rey may be the key to helping him overcome his guilt and becoming that hero we remember him being in the Original Trilogy.
Star Wars: The Last Jedi opens in theaters on December 15.