We're all familiar with the story of Luke Skywalker, right?
RECAP: Luke, fresh off the Tattooine moisture farms, meets an old warrior whose ally is the Force, trains about ten minutes, and ends A New Hope by trusting in the Force and blowing the Death Star to kingdom come. In Empire, still a novice, he gets his ass handed to him by a beast on an otherwise lifeless planet And receives a vision telling him to go to Dagobah.
Green as can be, he discovers the Jedi master Yoda and trains with him.
During training, he receives a vision of his friends suffering, so he prematurely ends his training to go find them.
It's of course, a trap, that leads to Luke dueling the Evil Darth Vader, who he discovers is his father after getting his ass beat down and losing his hand and a lightsaber. He escapes.
In Jedi, Luke orchestrates a successful breakout of his best friend from one of the most notorious criminals in the galaxy, defeats his band of thugs, returns to Dagobah to see Yoda off, chats with Obi Wan where he deduces Leia is his sister, and then eventually confronts Darth Vader, right after being totally willing to cut down an unarmed man who add of yet had displayed no threat, whoops Vader's ass, gets his father to turn back, and indirectly results in the Emperor being destroyed all while shrugging off raw hatred in lightning form after several minutes of torture.
HOLD THE FUCKING PHONE! You see a problem here? Our hero goes from one step above a guy who stumbles into good fortune into THE MOST POWERFUL WARRIOR IN THE GALAXY in the gap between Empire and Jedi.
Let's back up to Empire, when Luke first sees the vision of his friends at cloud city. Yoda was trying to convince Luke to stay and complete his training. Let his friends die. Even Obi Wan was unsympathetic towards Luke's concerns. Luke goes any ways, he's confident that Yoda has taught him the gist of things, but he gets his Frelling ass kicked. Remember, Luke thought several times the things they were doing in training were pointless. As far as Luke is concerned, this validated those beliefs. Even before he loses an arm, Vader tells him he's beaten. Vader is just toying with his food. It's after Luke lands a glancing blow to Vader's shoulder that he gets pissed and ends the fight. Luke got whooped. Then, Vader drops the biggest bomb of Luke's life. "Obi Wan never told you what happened to your father." Vader makes sure to say this, because as so many characters (even Vader himself) remind Luke over and over again "Your thoughts betray you" and Vader was sure to destroy the trust Luke had in Obi Wan. "I am your father" was the icing on the cake. Consider the trust gone at this point. Luke's "That's not true! That's impossible!" was just as much dealing with the fact Obi Wan lied to him as it was his father was Darth Vader.
So now, put yourself in Luke's shoes. You just spent god knows how long training in exercises you thought were pointless to begin with lead by a slightly annoying little alien who spoke in riddles and still got your ass kicked, you found out the man you revered as flawless straight up lied to you, and worst yet weren't the least bit sympathetic towards your friends who were in distress the last time you saw the, all while coming to grips with the fact that you couldn't save your best friend from being delivered into the hands of one of the most feared criminal warlords in the galaxy.
Damn. That's a bit dark, no?
The next time we see Luke, he's wearing a black robe. He's force choking people (an act no other good guy in the Star wars movies has ever performed) he's arrogant in his abilities "Beware my power," "I warn you not to underestimate my power," "free us, or die" (kinda sounds like Vader's "Join us or die" doesn't it?) . He makes quick waste of a Rancor. He's built a new lightsaber. After displaying all these new abilities and change in personality, he goes back to Dagobah for the first time since he left in Empire. He immediately knows Leia is his sister despite no clues in Obi Wan's speech.
Next, when he confronts Vader on the forest moon, he uses a tactic every single bad guy in Star Wars has used by using Vader's feelings against him. The final line is a straight up dagger into Vader's chest "Then my father is truly dead". Luke had Vader right where he wanted him by manipulating him emotionally, a trait seen across the board by bad guys in Star Wars.
Finally, when he fights his father on the Death Star in the throne room, he is fighting in a different style AND kicks Vader's ass.
Luke DID NOT GO BACK TO DAGOBAH in between Empire and Jedi, this is canon thanks to the conversations he has with both Yoda and Obi Wan. "Is Darth Vader my father" "Why Didn't you tell me?" etc…
So if he didn't go back there, to the only other Jedi in the galaxy, and he didn't go to Vader, who taught him to use these new powers he displays in Jedi? Who taught him what are clearly dark side tactics, who made him into a more powerful warrior than the fucking Chosen One?
I think Luke, in a truly dark place at the end of Empire, sought some one out who could teach him in the ways of the force, if even to tap into the darkside.Some one who could show him the power to save his friends and defeat Vader. Only one other force sensitive being was around: Snoke.