So on BS01 they had a contest for Star Wars; whoever could come up with the most crackpot theory would win a free Darth Vader set signed by the LEGO designers. I won the contest, with perhaps the dumbest theory out there- of course the final part has proven wrong at this point, but otherwise I think it holds together unless Rebels did something sneaky behind my back.
So the theory is: beards can make Jedi become ghosts. And when Luke (supposedly) dies in Episode Seven, he becomes a ghost.
It all started with Qui-Gon.
At the end of Episode One, Qui-Gon is rather viciously stabbed through the chest and killed. He is later said to be a ghost in episode 3. And he had a Beard.
Mace Windu was forced out of a window by Lord Sidious and died. He had no Beard, and a ghost he was not.
All the Jedi younglings were slaughtered by Anakin Skywalker, but no Beards were present and they did not become ghosts.
In Episode Two and Three, lots of poor, Beardless Jedi met their fate during the Clone Wars. And we never see them become ghosts.
Obi-Wan was the next ghost. He died in Episode Four at the hands of Darth Vader, stating "I shall become more powerful than you could ever imagine." And he had a Beard.
Now, you're probably saying "But Ki-Adi-Mundi had a Beard, and he died! Where's his ghost? And what of Yoda and Anakin? They were ghosts, but I never saw them have Beards."
Well, Ki-Adi-Mundi was unaware of the secret power of the Beard, and had not been trained to know how to use the power of the Jedi Beard, so he missed his opportunity. Plus, have you seen his Beard? It was not as Beardly as the Beardliness of the properly Bearded.
Yoda had no Beard, because he was incapable of growing a full Beard without taking about 500 years. And considering he only found out at the end of the Clone Wars, he was unable to grow one on such short notice.
So, by using the incredibly Beardly power of the two most powerful (and only) Ghost Beards in existence, Yoda was able to be a ghost thanks to the efforts of Qui-Gon and Obi-Wan.
Anakin was brought to ghosthood in the same way, through the powerful Beardliness of the former Jedi knights. The exception was made for him because of his short return to the good side of the Force, and you can't grow a Beard in a few minutes.
Now, there's been a lot of discussion on the figure in the snow in the newest Star Wars trailer, and nearly all evidence seems to point towards Luke. My guess is he will die, and his lightsaber will be passed on to Finn, who will defeat Kylo Ren. And Luke will probably become a ghost as well. But why would he do that?
He has a Beard.
"Strike me down, and I will become more powerful than you could ever imagine."