The end of The Force Awakens saw Rey heading to Ahch-To in hopes of recruiting Luke Skywalker into the Resistance, with Luke reluctantly agreeing to help the budding Jedi hone her abilities in The Last Jedi. He offered to teach her three lessons, yet the finished film only included two. SlashFilm has recently revealed the details of one of the film's deleted scenes, which confirmed the third lesson.
***WARNING: Spoilers below for The Last Jedi***
In a scene that would have chronologically taken place after Luke revealed the details of Darth Sidious to Rey, the duo both notice boats approaching Ahch-To, with fires raging on the island where the boats presumably landed. Luke informs Rey that these are bandits that regularly travel to the island to murder the Caretakers who live there.
Rey, understandably, wants to protect those who ensure the remains of the Jedi Temple stay intact, yet Luke tells her a true Jedi Knight wouldn't interfere in the "balance," despite people getting hurt. This doesn't sit well with Rey and she rushes to where she sees the fires blazing, with her Force abilities making her approach even more ferocious.
Upon her arrival, Rey realizes that the Caretakers have faked the encounter and are celebrating around a real fire. The entire story was an elaborate ruse to test Rey about how she'd react to the idea of someone being in danger. Luke then confirms that this was another important lesson she needed to learn, which is that she is exactly what the Resistance needs, a person willing to save anyone at all costs, rather than a former Jedi who has chosen a life of solitude.
The two share an emotional interchange, with Rey expressing her exasperation at seeking help from the Luke Skywalker of legend, not the man who would settle for injustices existing in the galaxy.
One reason the scene was likely
Another reason is that, compared to lessons like the Force not belonging to anyone, Jedi or Sith, and that the Jedi Order must come to an end, this lesson wasn't quite as succinct or easily digestible.
The Last Jedi is in theaters now.