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              There's a price for everything               
                        -by Zalmax-                        
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          When Royals refuse to agree to disagree          
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When Royals refuse to agree to disagree
“We said no! The Sun and the Moon are opposites. Thee can not deny us the right to refuse. To be different, is part of who we are, and as a Princess of the Moon we must follow our calling.”
“I implore you to get along with the times. This isn’t the same Equestria you once knew. The last millennium has seen a lot of changes.”
“Silence sister! We wilt not succumb to these modern rituals thou so boldly claim are royal duties. This is not the reason we have returned. Our purpose in Equestria is to walketh through dreams, and to save ponies from the dreams that hurt. What thee demand of us is wrong. So very wrong.”
“Please calm down and think about it. It's not that hard, and it's a very important part for the ponies who attend. It’s something they've been looking forward to for a long time.”
“Never! We will not be swayed by thine pleading eyes or the dripping molasses in thy voice. The entire premise is massively unfair. Not all ponies in Equestria can partake in the event – only a few select. What happens during this ritual brings fortune to some, and excludes all others. We will not contribute to unjust treatment of our subjects.”
“Please be reasonable, and think about it. When you walk through Canterlot you do it all the time. Why can’t you do it now?”
“It’s different. Anypony in Equestria can approach us as we stroll through Canterlot. Only a few select are invited to this event, while the rest – are forbidden from entering.”
“It’s such an easy task, with the ponies patiently standing in line; some have spent months planning, what to say, how to do it, and even practiced it. It’s not too much to ask that we both attend to fulfill their wishes. For many ponies it will be the first time in their life they can have a personal interaction with both the Princess of the Sun and the Princess of the Moon.”
“Thee should know that we fulfill many dreams in ours line of work, but this type of fantasy is not healthy. We have seen their dreams, and this ritual tricks them into believing that dreams can come true. We have seen enough bad dreams to know that it is best for ponies if they do not believe that. We should all keep reality and dreams apart, or they will end up with a Tantabus going rampant through towns. We do not want disaster to befall us.”
“I’m sure we can do this without any catastrophe to fall upon Equestria, and it would bring so much joy and glamour to the event. It’s just a personal greeting, that’s all.”
“Greeting ponies should be done from a balcony. It is the only way we can guarantee that everypony can attend, and not just those with a ticket. Greeting from a balcony is also fair, as everypony receives the same greeting and the same amount of attention. This is the ancient tradition for how a princess should show gratitude to her minions, and we are staunch believer in the old customs, where fairness ruled supreme and everypony were equal.”
“So you insist all ponies are equal?”
“Except princesses! They should be on a balcony. And wipe that smirk of thy face.”
“But you’ve said it yourself that part of being a princess is to ensure Equestria prospers. Bringing happiness to those who attend will bring prosperity to their lives.”
“Prosperity is when they can fill their bellies with fruit and vegetables. Prosperity is to spend the day and night with peace of mind. Prosperity is knowing that the immortal alicorns use their eternal wisdom to ensure their safety. Do not muddle the royal duties with sampling cake and one-on-one gossip.”
“But the Grand Galloping Gala is our most celebrated event, and it’s only once a year. Please, stand by my side and let them great each of us in turn, as we welcome them to the event.”
“Our mind is made up. We wilt not change it. There are some things in this world that just is not right, and it is our royal duty to refuse when we see unfair treatment that risks bringeth jealousy and envy to ruin Equestria.”
“Please?”
“No.”
“Pretty please?”
“No!!”
“I'll share you all of my cake?”
“We can get infinite cake by visiting thine dreams.”
“I'll put a restraining order on Prince Blueblood?”
“I am listening...”
“For the next year, he’s not allowed to decrease distance if within fifty strides from you.”
“Sister! You have once again used your cunning vileness to make me realize that there is a price for everything. I will stand at the stairs and greet each pony with a corresponding phrase and a smile.”
“I'm so happy! This will be the best Grand Galloping Gala ever. Dear Luna, I love you, even if you can be a bit stubborn.”
“It will be terrible. But feel free to keep hugging me. It is, after all, part of my duty as a sister, royal or not.”
The End
Rule #2 in this contest gave a minimum word limit of 500 words, but the good rules of FimFic refuses to let me publish it with just 851 words, so I added this text at the bottom as a workaround. Hopefully moderators will not delete my story because of my sneaky tricks, but in case it happens – I doubt anyone will miss it. At least I know I won’t.  
To stretch out the dialogue, by adding some filler paragraphs, didn’t feel natural for how the discussion went along. It would just be filler words without a clear purpose and detract from the main stance in each participant’s point of view and thereby remove the energy in their discussion.  
Having Princess Celestia explain how fantastic the event is wouldn’t really impress a Princess Luna who has been around for thousands (?) of years so there was no point in having them bring up topics that would be well known to both of them, instead of focusing on the stubbornness of Luna.