Han Solo telling Rey and Finn that Luke didn't just leave because he felt guilty over Kylo Ren turning to the dark side, but that he was looking for the first Jedi temple. Also, and I'm not sure if this was officially confirmed or not, but the map both R2-D2 and BB-8 had didn't just tell where Luke was, but also possible locations of Jedi temples. And remember, the First Order has incomplete information gathered from Sith and Imperial archives of this same map. So right away, you have a literal race between the good guys and the bad guys to find the first Jedi temple, which of course would contain all the secrets of Force, etc.
Question is, why would Luke want to look for this first Jedi temple? Well, the assumption is that, after the fall of Ben Solo, Luke would want to seek out this ancient Jedi knowledge to find out where he went wrong, right? Maybe, in Episode 8, this is also what Rey assumes as well. However, what if Luke is looking for these Jedi temples not to learn new knowledge but destroy whatever he finds?
Maybe, after Ben Solo turned to the dark side, Luke came to the conclusion that no one, Sith or Jedi, should have knowledge of the Force, believing that anyone trained in its ways, even for the best of intentions, makes them more susceptible to the dark side. Luke can even point out how his father was strong in the ways of the Force and was already good, but it wasn't until he was trained as a Jedi that he became tempted to turn to the dark side and became Darth Vader. His sister Leia, like him, is also strong in the Force but was never trained as a Jedi and yet she is squarely on the side of the light. He could even say, "Maybe if I just left Ben alone and let him find his own path, he would not have become Kylo Ren."
This also makes Rey's role even more clear as she can be the one to try and bring back hope to Luke, to remind him that people can choose to be good or evil even without knowledge of the Force. Maybe she can even tell him that the problem isn't about knowledge but feelings. The Jedi sought to use the Force to control their own feelings, and while they were more disciplined and at peace, the result was they became too rigid and dogmatic, unwilling to be open to change or life's pleasures. By contrast, the Sith went in the other extreme, following their passions in the belief it made them stronger in the use of the Force, but also allowing feelings such as anger and fear--which are not actually evil by themselves--to run amok and unchecked. In other words, through Rey, Luke can be reminded of Yoda taught about the Force--it should not be something you control or let it control you; it should be your "ally," or rather a friend, something you work with.