How we cite our quotes: All quotations are from Star Wars: Return of the Jedi.
Quote #4
DARTH VADER: The Emperor has been expecting you.
LUKE: I know, father.
DARTH VADER: So, you have accepted the truth?
LUKE: I have accepted the truth that you were once Anakin Skywalker, my father.
DARTH VADER: That name no longer has any meaning for me!
LUKE: It is the name of your true self; you've only forgotten. I know there is good in you. The Emperor hasn't driven it from you fully. That was why you couldn't destroy me. That's why you won't bring me to your Emperor now.
Given that Darth Vader delivers Luke to the Emperor right after this, it looks like the joke's on Luke at this point. Still, Luke keeps the faith.
Quote #5
LUKE: Search your feelings, Father, you can't do this. I feel the conflict within you. Let go of your hate.
DARTH VADER: It is too late for me, son. The Emperor will show you the true nature of the Force. He is your master now.
LUKE: Then my father is truly dead.
Is Luke engaging in a little reverse psychology here? He's practically daring Anakin to ditch the mask and man up.
Quote #6
DARTH VADER: His light saber.
THE EMPEROR: Ah, yes. A Jedi's weapon, much like your father's. By now you must know that your father can never be turned from the Dark Side. So will it be with you.
Given all that they have in common—their Jedi training with Obi-Wan, their robotic right hands, their love of wearing all black—part of Luke's motivation to save Anakin is to save himself. If it's Luke's fate to end up like his dad, he wants his dad's story to have a happy ending.