He was separated at birth from his sister, who he didn't even know he had, and raised by an aunt and uncle who struggled to keep their moisture farm afloat in a sea of sand, while dealing with a moody and whiny teenager who would rather be getting power converters at Tosche Station. The area was plagued with murderous sand people (who he is likely unaware that their compatriots killed his grandmother), Jawas with dubious motivation, and slug-like gangster Hutts. Oh, and let's not forget Greedo, who may or may not have shot first.
He came upon a droid on a mission, with a message from the sister he didn't know he had. This led him to find out that his whole life was a lie, the old man he thought was just a lonely old hermit was actually a famous Jedi who had fought in the legendary Clone Wars, his farm would be decimated, he had a twin sister with a much cooler title, and his father was the very person the old hermit told him killed his father. The man he had always admired and wished he'd known. Talk about disillusionment!
At first, he trained hard to fight the evil that brought his father to a fraction of the man he had once been. But after triumphing over the Sith and actually losing his father in the process, his new role of training a new generation of Jedi ended in disaster, with his own nephew going on a murdering rampage. Like Yoda and Obi-Wan before him, Luke went into hiding. The years passed, and Luke waited on a far away island on a lonely planet.
Or did he?
What if all the tragedy in his life led him on a darker path than we realized? What if he became a dark lord himself? He once said he was a Jedi like his father before him. What if he decided to become a Sith like his father before him as well? Maybe one who would continue his instruction of his nephew in the ways of the Force. But now in holographic form, from a distance. The giant hologram would add to his air of mystery, and he could instruct people all over the galaxy from the comfort of his throne on a remote island.
But wait, you say. He doesn't look like Snoke! Ah, but he has had years to figure out and study the ways of the Force. Who knows how much the Force can do? I'm sure the movies don't even scratch the surface. Maybe people with enough midichlorians can change their appearance at will.
You may object that we see Luke and Snoke in two different places. First of all, he isn't actually present in the room where he talks to Kylo Ren and Hux. He is there in holographic form. Secondly, in the prequels, we get to know Senator Palpatine pretending to fight for good, and we also see Darth Sidious guiding the bad guys in the dark side of the Force. They don't reveal until later that they are the same person. Maybe this is another case of that. Perhaps they will reveal that Luke and Snoke are the same person.
Lastly, both names are one syllable and end with "ke"... Highly suspicious.