10 Fascinating Theories About Star Wars: The Last Jedi
2. R2-D2 Holds The Key
R2-D2, in low-power sadness mode after Luke Skywalker’s disappearance, perked up at the end of The Force Awakens and proved to contain half of the map to his master's location. He was also seen in the traumatic flashback when Rey found Luke’s lightsaber.
There’s no explanation why R2 should contain this information or why Luke didn't bring the ever-useful pepper pot along with him in his exile.
One explanation is Luke has given R2-D2 some part in whatever plan he has concocted, and primed R2 to reveal his location only after Starkiller Base was dealt with.
Whatever Luke's plan, this theory makes R2-D2 a key component of it just like the droid was crucial to A New Hope. R2 is programmed to act as Skywalker's proxy in his absence, like a sleeper agent parcelling out intelligence to the Resistance whenever it suits Skywalker, for good or ill.