Some believe that when you die and go to heaven or what not, you can apper as a ghost when you felt happiest.
Ben died helping his freinds out in the Death Star, knowing he had foung the 'new hope'. Thats the only reason he spent all his life on Tatooine. His mission is complete. He's done his bit. He's redeemed himself for Vader.
Yoda, simular in the way that the 'new hope' has made his way to Dagobah, with Yoda waiting all this time for Luke. Knowing that Luke is the only real way this shit will all end.
Then Anakin. Who died twice in a way. As old Anakin, he died happy. It was finished. They had won. With the help of his son, who up untill recently he thoght dead(Maybe? - 3
) had saved him. Killed that soab Palpatine and restored freedom to the Galaxie. But he also died once before. Was mureded by Darth Vader. When Anakin seeced(sp?) to exist.So why I say this change does actually make more sense is this, if you watch then in order 1- 6, is:
As gohst's we get to choose how we look. For Ani to look like his younger self is as simple as this. Thats how everybody apart from Luke, knew him when he was good. The duel we'll see in 3, thats the last time Anakin Skywalker was seen. From then on he becomes Datrh Vader.
Only when Luke saved him, all he saw was this tired, sad looking old man.
So when Ani becomes a ghost, he chooses to look like the person they all knew and loved when he was still Anakin and not Vader.
Luke must know this too, so thats why he's smiling. He knows that he's done his bit too.........
EDIT...............
EDIT...............
While i was writing all this shit, there were 2 more posts!
So it looks like I'm repeating what someone else said. I'm not