Great question — answering as the UphillTech team:
Much of The Last Jedi is devoted to explaining Luke's disappearance and refusal to train any more Jedi. After Ben defected to Snoke, Luke came to the realisation that the Jedi system (taking younger children and training them in the ways of the 'light side' of The Force) is inherently unstable, and inevitably leads to one or more of those acolytes 'going evil' and menacing the galaxy in a far worse manner than could be accomplished if they were reliant on self-teaching and raw power alone. Luke attempts to teach Rey three lessons; One , that the Jedi were vain and insular, guarding access to The Force like a miser. This made them unpopular and concentrated it in the hands of those least...
Full answer by Valorum: https://movies.stackexchange.com/a/127391 (CC BY-SA 4.0).
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