Every Jedi master takes a different approach to the young Force-sensitives who seek them out for tutelage, and Luke Skywalker is no exception. As great as it was to see what sort of teacher Luke would be, there was one part of his getting to know Rey that was truly disturbing, and in the worst possible way.
When Rey finds Luke on the remote island of Ahch-To, he's the galaxy's most secluded Jedi. Disillusioned at the role he played in Kylo Ren's creation, Luke withdraws into himself and hides from the rest of the universe in the ruins of the first Jedi temple. At first, Rey finds that Luke's become something of a curmudgeon in his old age, preferring the company of Ahch-To's porgs and the stern Lanai who maintain the Jedi's ancient structures on the island.
Though Luke repeatedly tells Rey that he has no interest in training her to be a Jedi, she persists in trying to convince him that the Resistance desperately needs his help. Like any grumpy sage worth his salt, Luke is resolute in his refusal to help Rey; in return, she takes to following him around the island, hoping to wear him down, or at least learn something from his daily routine.
Rey sees Luke meditating, hiking, and fishing and he does eventually open up to her somewhat, but not until he's made sure she's seen one of the oddest things in Star Wars history. While running his errands, Luke makes a pit stop along the coastline to meet up with and milk a Thala-Siren, one of the new creatures introduced in The Last Jedi.
A Thala-Siren Image: Wookiepedia
There's nothing inherently weird about this, but the way that Luke goes about milking the Thala-Siren is decidedly odd. The Jedi squats down between the marine animal's legs, gets face to face with its teats, and begins to coax jets of blue-green milk out of them. Then, he immediately begins to swig the fresh milk out of a bottle, making direct eye contact with Rey as the milk drips down his grizzled, bearded face.
To listen to the way milk enthusiasts talk about drinking farm fresh milk, you'd think the stuff was laced with highly addictive drugs, but Luke's Thala-Siren stunt was on some next level creepy shit. Rey's mortified by Luke and honestly, can you blame her? How would you feel if a strange old man wearing (glamorous) rags led you on a wild goose chase around an island and then sloppily drank the milk of a sea creature while licking his lips as if to say "That's right; this is what it means to be a Jedi. Still want in?"
That Rey didn't give up right then and there is a testament to her fortitude and honestly, I can't say that I would have had her strength. Between his beard and windswept hair, old age has turned Luke Skywalker into something of a wild man, and there's nothing quite as disconcerting as watching a wild man with a crazed look in his eye guzzle the liquid he's just milked out of an animal while he makes direct eye contact with you. What's more, Thala-Siren teats look an awful lot like humans breasts, making the entire sequence of events that much creepier.
Depending on how you look at it, Luke was either trying to drive Rey away by being weird as hell, or that's just the man that he became later on in his life -- a bitter hermit ashamed of his past and addicted to suckling sustenance out of poor, innocent mammals who were just trying to relax by the ocean.