In the phantom menace we learn that Anakin Skywalker who would one day become Darth Vader, was the chosen one from a prophecy who would bring balance to the Force.
Some say that Anakin wasn't really the chosen one from the prophecy, because he turned to the Dark Side. And that even though he did defeat Darth Sidious, he couldn't have done so without Luke.
But he did defeat Darth Sidious. And he did bring balance to the Force, even if it didn't last. Therefore the prophecy did come true and Anakin was the chosen one.
Anakin did fulfill the prophecy, and bring balance to the Force. It just didn't last.
As you know, Anakin became a Force ghost at the end of the Return of the Jedi.
As a Force ghost, it is only logical to assume that Anakin has been watching the happenings in the galaxy, ever since. The same way Obi Wan did. And Yoda who even appeared as a Force ghost in the Last Jedi.
Anakin most likely saw everything that happened in the galaxy after the return of the Jedi. He probably watched the balance in the Force slowly crumble, until it was gone again. He probably felt that he had failed, that he not fully brought balance to the Force. That he had never really accomplished that task in his life.
When darkness rules, light always rises to face it. When light rules, darkness always rises to face it.

"Darkness rises, and light to meet it."-Snoke, the last Jedi.

Every time one side of the Force is defeated, it always rises again. It never brings balance. It has been a constant pattern in the Force. One side rules, then the other side rises to defeat it. And again and again. It could continue forever with only brief moments of balance that never lasts.
The only way to bring true balance is to set aside both the Light Side and the Dark Side, leaving only the Force. The only way to do this is to bring the Light Side and the Dark Side together. The answer is not destroy both of them. Because the only way for there to truly be balance, is to have both the light and the dark sides of the Force working together.
But now that the Force is no longer balanced, as the chosen one, Anakin would see it as his duty to bring balance to the Force.
What if Anakin was behind EVERYTHING?
What do I mean?
What if Anakin was the one who caused the connection between Rey and Ben?
Both Rey and Ben have something that once belonged to Anakin Skywalker.
Ben has Darth Vader's helmet, which was the part of him that was on the Dark Side.

Rey has Anakin Skywalker's lightsaber, which was the part of him that was on the Light Side.

Ben, or Kylo Ren is on the Dark Side.
Rey is on the Light Side.
What if they were influenced by the objects that once belonged to Anakin? What if Vader's helmet called Ben to the Dark Side, and Anakin's lightsaber called Rey to the Light Side?
When Kylo Ren was talking to Darth Vader's helmet he said: "show me again the power of the Dark Side, and I will finish what you started."
"Again." Which means that he had most likely had a vision in through the Force while in the presence of Vader's helmet.
I think that Ben saw Rey in his Force vision, and that was why he seemed to recognize her.

When Rey touched Anakin's lightsaber, she had a vision in the Force. She saw Kylo Ren (Ben)

If Ben had a vision while holding Vader's helmet, and Rey had one while holding Anakin's lightsaber, Anakin may have been the one who caused the visions. He may have used the two items that had once belonged to him, one from the Dark Side, the other from the Light, to communicate with two of the last Force users. Using them to bring balance to the Force.

Anakin might have been the one who called Ben to the Dark Side, using the helmet of Darth Vader to do so. He may have also called Rey to the Light Side, using his old lightsaber.
We didn't get to see Ben find Vader's helmet, so we can't say for certain that it called him. But from the Force Awakens, we know Anakin's lightsaber called to Rey. So it is possible that the helmet also called Ben. It would make sense, how else could he have found it?
Why would Anakin call Rey to the Light Side, and call his own grandson to the Dark Side, after what happened to him? Because Ben already had the Dark Side within him. And Rey already had the Light Side within her. But Ben also has the Light Side within him, and Rey also has the Dark Side within her.
Both Rey and Ben are lonely and feel abandoned by their parents. Because they are so lonely they can understand each other. Rey can see past Kylo Ren, to see Ben. She can see past the darkness to see the light deep within. Ben can see past the light in Rey to see the darkness deep within. They each see themselves within the other. Yet have the other within themselves.
That is why Anakin chose to use them to bring balance to the Force.
But most importantly, because of their compassion for each other.
As I said in "theory #5" compassion is the only thing strong enough to bring balance to the Force. As a Force ghost, Anakin knows that compassion between the light side and the dark side is the only thing that can bring balance to the Force.
Now that Snoke and Luke are gone, Rey and Ben are really the last Force users left. There are other Force sensitive beings, but they aren't Jedi or Sith , they haven't been trained to actually use the Force. With Rey being the last of the Light Side and Ben being the last of Dark Side, their compassion for each other will be brings balance to the Force.
I think Anakin might have been the one who caused the connection between Rey and Ben. Snoke claims that he "bridged their minds." But Snoke is a lier, so we can't believe anything he says.
According to Star Wars resources, Rey and Ben had a connection in the Force before the Force Awakens. Snoke didn't know anything about Rey until Ben told him, after he kidnapped her. So how could he have caused the connection?
No one can cause a connection in the Force, only the Force itself can. As a Force ghost, Anakin is a part of the Force now, so he could have caused the connection.
He could be trying to bring Rey and Ben together, and in so doing bring balance to the Force. Rey is the Light Side, Ben is the Dark Side.

"Light, Darkness, Balance."- Rey, The Last Jedi.

If Anakin brings them together, they could bring balance to the Force. And Anakin would technically be fulfilling the prophecy.
If anyone caused the bond, it was Anakin. He knows that Rey and Ben's compassion for each other has the power to bring balance to the Force.
It might seem odd to think that Rey and Ben have compassion for each other, since they are enemies. But they do.
Even Snoke could sense it:

"You have compassion for her."
Snoke, the Force Awakens

While Ben did appear to have compassion for Rey in the Force Awakens. It was even more obvious in the Last Jedi. He listened to her, and told her that she wasn't alone.

Then he killed Snoke to save her.
The real question is rather or not Rey has compassion for Ben.
The answer in my opinion is that she does. She spared his life twice. She had the opportunity to kill him, twice. He is her greatest enemy, yet she spared him. If that isn't compassion, I don't know what is.
She went to the First Order to help him, to bring him back. Even though she knew it was dangerous. She risked her life to save him from the evil that had taken hold of him.

Most importantly she was able to look past Kylo Ren and see Ben. To see the good in him.

I can imagine that Anakin was pretty disappointed when Kylo Ren and Rey started fighting over the lightsaber, instead of working together to bring balance to the Force.