"Master," Darth Vader began. "I am curious."
"...troubling, but go on," Palpatine replied.
"What would happen if you died?" Vader said.
Palatine considered his answer.
"Why do you want to know?" he replied.
"I am curious," Vader responded. "We covered this."
"Well," Palpatine said. "In order to prevent anyone from my family assassinating me, the legal results are meant to be an enormous snarl. Familicide is the main cause of death among Palpatines, statistically speaking, over the last hundred years."
"I mean more in terms of what happens both to the galaxy, and also… physically," Vader explained. "I was going to see if it happened through old age, but as far as I can tell you have only aged once since I met you and that was all in a single burst."
"You were going to see if I would die from old age?" Palpatine asked.
Vader shrugged. "I did not see a way to prevent it," he replied.
"Well, for your information, I intend to simply not die," the Dark Lord of the Sith told Vader evenly. "I am simply built different."
Vader looked his master up and down.
"You are not," he said. "You are not built at all. I am built different, because over fifty percent of my body weight is cybernetics of non-standard design."
"It is a figure of speech, Vader," Palpatine told him. "It is popular with the 'youth' of today."
He looked momentarily thoughtful. "Or, at least, so I am told. My advisor Jade told me."
"I see," Vader said, who didn't. "Though I am fairly sure it is possible for Sith to die. I caused it to happen, after all."
"Oh, Tyrannus wasn't a proper Sith," Palpatine muttered, dismissively.
"...he could do Force Lightning," Vader objected. "I cannot do Force Lightning."
"Then maybe you are not a proper Sith either," Palpatine said, then shrugged. "But the fact that he died while still the Apprentice showed that he did not have what it takes to succeed as a Sith."
Vader tilted his helmet slightly, and Palpatine waved his hand.
"Don't even think about it," he said.
"I can think about it if I want," Vader replied, a bit testily. "But, as I say, I am curious. What would happen?"
"To the galaxy…" Palpatine mused, considering it as a thought experiment. "Well, I suppose that depends on the exact details, but I suppose Amedda would try to seize power. He's pathetic, though. And I suppose you might try to command the military, but you're no good at politics, so you would have to rule through fear."
"I am good at fear," Vader said.
"True," Palpatine admitted. "But it's academic, anyway. I have a plan in place that would result in the galaxy being burned to the ground if ever I die. And I have another plan to clone me so I can possess my own clone and return to life."
Vader coughed, which was quite a feat for him.
"What?" Palpatine asked. "What is it now?"
"You don't think you might want to pick just one of those, Master?" he asked. "Unless you want to return to life in a clone body in a galaxy that has just been burned to the ground?"
Palpatine shrugged.
"I don't intend to test either of them," he said. "Like I said. I would simply choose to not die."
"I really don't think that's an option," Vader replied, thoughtfully. "If it was, I think everyone would do it."
"Built different," Palpatine reiterated. "Keep up, Vader."
"Then what about physically?" Vader went on. "If Dooku wasn't a proper Sith, that is. All that happened with him is that his head came off."
"Well," Palpatine began. "I do have an absolutely enormous amount of Dark Side energy suffusing most of my bodily parts, to keep me in good shape as otherwise I would have suffered total organ failure."
"Total organ failure… from what?" Vader said.
"The Dark Side energy suffusing most of my body parts," Palpatine answered. "It also renders me immune to poison… hmm… what would happen if I died… well, I suspect that all that Dark Side energy would explode outwards, conveniently also killing whoever it was who had managed to kill me."
He looked thoughtful. "Now there's an idea. Perhaps if I do it properly, I can skip the clone phase and just possess whoever it is who killed me. It would be nice to be in a younger body."
Vader raised one of his hands.
"What about if Yoda had killed you?" he asked.
"Academic," Palpatine pointed out. "Since I chose not to die. Really, this conversation would be a lot shorter if you'd just accepted that answer at the beginning… it's youth language again, but if I understand what Jade was saying right the simple way to put it is that I identify as immortal and you should respect that."
Vader nodded slightly.
"...you said clones," he pointed out. "Do those clones exist already? Or are they going to be started later?"
"Why does that matter, Vader?" Palpatine asked. "Do not think you can destroy my contingency plans. They are far out of your reach."
"I'm more wondering something else," Vader defended himself. "About your clones. Do they come out as old as you?"
Palpatine blinked.
"What?" he asked.
"Or do they come out young?" Vader went on. "Because thinking about you as a fifteen year old is giving me the heebie jeebies, but imagining a one hundred and thirty seven year old fresh grown clone is a bit weird as well."
"You never talk like this around anyone else," Palpatine said.
"I have an image to maintain," Vader replied. "Fear. And so on."
Palpatine had been processing something else.
"A hundred and thirty seven?" he asked. "Vader! I am eighty three."
"Really?" Vader asked, sounding honestly surprised.
He paused.
"...you look good for it. You know. Considering."
"But to answer your question," Palpatine went on. "Of course they come out the same age! What would be the point of coming back looking completely different?"
"Couldn't you just be in a younger body now?" Vader asked. "By cloning yourself a younger body, and possessing that? I'm just wondering why you wouldn't take that option."
Palpatine considered the question.
"Spite," he said, eventually.
"...is that the only answer?" Vader said, after several seconds of silence.
"Yes, pretty much," Palpatine confirmed.
AN:
It's his reason for everything.