As of the Force Awakens we know that after the destruction of his new Jedi Order by his prized pupil Luke Skywalker abandoned the galaxy to itself. At first glance this could be the actions of a broken man, perhaps one suffering with PTSD, who has ran away from all his issues, but maybe there is more? I mean if your going to run away from all your problems a remote island on a remote planet in a remote star system seems a little illogical after all: “Where does a wise man hide a leaf? In the forest. But what does he do if there is no forest? … He grows a forest to hide it in.” Luke being the only living person on a planet seems to me like he would stick out like a sore thumb, though he may have learnt his hiding skills from Obi-Wan who was kind of terrible at it.
The school of thought I subscribe too is that Luke is not in exile but in retreat. He has sensed Snoke and realizes that this is an ancient and evil threat, one that he is not equipped to battle. A threat that he is looking to stop by studying ancient Jedi texts, ones that may lead him down a path of discovery that will give him the strength to vanquish Snoke.
Or there is another school of thought. What if my previous idea about Snoke needing to consume powerful force users to survive was correct and Luke new that if he stayed around he would just become a bigger threat to those around him, so he retreated to become more powerful to fight Snoke and destroy him. This is all purely speculation but its interesting to think about.