2 parts Complete Obi-Wan Kenobi didn't die on the Death Star. Instead, he continues to train Luke.
On a Jedi field trip to a Sith temple, they are not the only ones around. They find a certain Sith, caught deep within a trap of the temple; in which the victim's desires are used against them, creating a perfect dream of everything they want. To remove the Sith and drag him back to the Alliance a prisoner, Obi-Wan and Luke must delve into this vision to drag their quarry out.
AKA: Vader has a happy vision for once in his life, so Obi-Wan and his son team up to put an end to it. Because if our poor Ani is not dreadfully depressed for longer than a second the damn universe will explode.