In The Empire Strikes Back, Vader says to Luke:
Vader: If you only knew the power of the dark side...
And he says it in a way that's obviously advertising its appeal to Luke.
However, in Return of the Jedi, Vader says:
Vader: You don't know the power of the dark side... I must obey my master.
Do Vader's feelings on the dark side change between ESB and ROTJ? If so, what brought about (or would bring about) such a stark and more or less sudden change?