Darth Vader: Obi-Wan once thought as you do. You don't know the power of the dark side.
Yoda: When 900 years old you reach, look as good you will not.
Darth Vader: If you will not fight, then you will meet your destiny!
Luke Skywalker: Never. I'll never turn to the dark side. You failed, your highness. I am a Jedi, like my father before me.
The Emperor: If you cannot be turned, then you will be destroyed.
The Emperor: If you will not be turned, then you will be destroyed!
Han Solo: I have a bad feeling about this.
Luke Skywalker: You told me Vader betrayed and murdered my father.
Obi-Wan Kenobi: Your father was seduced by the dark side of the force: he ceased to be Anakin Skywalker and became Darth Vader; when that happened, the good man who was your father was destroyed. So, what I told you was true...from a certain point of view.
Darth Vader: You cannot hide forever, Luke.
Luke Skywalker: I will not fight you.
Darth Vader: Give yourself to the Dark Side. It is the only way to save your friends. Yes, your thoughts betray you. Your feelings for them are strong. Especially for...sister. So, you have a twin sister. Your feelings have now betrayed her too. Obi-Wan was wise to hide her from me. Now his failure is complete. If you will not turn to the Dark Side, then perhaps she will.
Luke Skywalker: [ignites lightsaber] NEVER!
Han Solo: I think my eyes are getting better - instead of a big dark blur I see a big light blur.
Answer: The Jedi are shown to have something of a blind spot in regards to matters of the heart. Note that when Luke confronts Obi-Wan over lying to him about his father's fate, Obi-Wan's response is haughty and defensive, and gives Luke nothing in terms of regret or apology. They're focused on their mission, not on how Luke feels. Why waste time, in their eyes, telling Luke about his mother? If they had their way, he wouldn't even know about his father. The prequels would make this more explicit, showing that the Jedi are conditioned from the beginning to let go of all "passions" because they could so easily be corrupted, and their inability to understand Anakin's emotions just contributes to his downfall.
TonyPH