I have seen a lot of people asking how Rey went into his mind when she didn't know anything about the Force.
This is my theory:
Unlike anyone else Kylo has looked into the mind of, he didn't just want to find the map, he wanted to learn more about her. Because he wanted to learn more about her his mind was open to her. She simply went through the open door. He unknowingly welcomed her into his mind.
I think that because he saw so much of himself in her, he unknowingly shared things about himself with her.
According to Star Wars resources Rey learned about the Force by seeing Kylo Ren's training. He later offered to teach her. He may have sensed the Force within her and had a desire to teach her how to use it from the beginning. Because of that desire he might have subconsciously showed her what he knew.
If Rey saw his training it would explain a lot of things. How she was so strong with the Force so quickly, and how she was able to beat Kylo Ren so easily. If she saw his training she would know his fighting patterns and be able to counter act them.
But I have a farther theory about how Rey was able to get into his mind.
I think that Rey and Ben have been connected through the Force their whole lives.
There are several things that point to this.
One
When Kylo Ren learnt that FN-2187 and the droid were helped by a girl he was surprised. He asked "what girl?" As if he knew of a girl.
I think he had had visions of her in the past. I think he saw her in the past, her past as a child, (which may have even been the present at the time when he saw it.) But I also believe that he saw parts of her future.
These are the parts of her future I believe he saw :
I believe he saw her wield the Skywalker lightsaber.
(In the novelization when the lightsaber flies past him to land in Rey's hand he says: "it is you." As if seeing her wield the lightsaber confirmed something about her)
I believe he saw her fighting against him in the forest...
...but I also think he saw her fighting at his side in the throne room.
He may have believed these to be two different possible futures. Either she would be the enemy who would destroy him or the alley who would save him. And that's why he was so surprised when he heard about a girl, he knew it must be the girl from his visions.
Two
In the novelization when Rey saw Kylo Ren, it said that she remembered him from a dream or a nightmare.
Three
In both the novelizations and the picture books it says that Rey and Kylo Ren shared a mysterious connection.
Four
Kylo said that the lightsaber belonged to him. What if it was Rey's connection to him that caused the lightsaber to call her.
When she touched the lightsaber she saw a series of visions.
She saw Kylo Ren in the forest.
She saw Kylo and knights of Ren.
She saw Luke after the Jedi academy was destroyed.
And in the novelization she saw Ben as a boy.
Think about it, most of these were his life, his memories.
Except one, when Rey's parents abandoned her. Or was it?
What if that was also one of Ben's memories. If they had a connection through the Force he may have had a vision of Rey being abandoned by her parents.
~ Rey stood their and watched herself as if she were looking through someone else's eyes.
~in the last Jedi Ben knew who Rey's parents were. What if he had known all along, because he had watched them leave her in a dream or a vision.
Four
Their connection grew stronger in the last Jedi, and while Snoke claimed that he bridged their minds, no one can create a Force bond. The fact that Rey and Ben saw each other at the end of the movie after Snoke was dead proves that he was not behind their connection.
This connection would explain a lot.
Like how Rey was able to go into Ben's mind when he was looking for the map. If they already had a connection then when he looked into her mind the connection grew stronger and she was able to go into his mind.