We...Actually Don't Know
Starlight Glimmer happily trotted down the streets of Ponyville, as Celestia's sun began to dip behind the mountains. In her magic, she held her prized kite, which she'd spent the last few hours flying alongside Maud Pie. Overall, she'd had a lovely, relaxing day, and she was looking forward to dinner with Spike and Twilight back at the castle, followed by a peaceful sleep.
However, once she'd reached the crystal building itself, Starlight discovered that her desires might not pan out that way. For one thing, Trixie's wagon was parked right outside the doors, and for another, the showmare herself was nowhere to be seen. Starlight immediately groaned, remembering what had happened the last time Twilight and Trixie were in the same room. She silently prayed history would not repeat itself.
Climbing the stairs, Starlight gripped the door handle in her magic, and swung it open, revealing an empty foyer. She glanced around, until her sensitive pony ears picked up what sounded like arguing. Her hoof immediately made contact with her face, and she violently shook her head.
"Why does this keep happening?!" she complained to no one in particular.
A door down the hallway opened, and out came a familiar purple and green dragon, who scurried up to Starlight as soon as he noticed her.
"Starlight! Oh, thank Celestia!" he panted. "It's worse than ever this time! I'm surprised they haven't started shooting magic beams at each other yet!"
"That bad, huh?" asked Starlight. Spike nodded vigorously. "What set them off this time?"
"No idea," replied Spike. "Trixie just came in, and they both said they'd be civil, and then, two minutes later, they started yelling."
Starlight sighed again. "I'll go defuse this. Could you make some snacks so we have something to eat while we cool down?"
"Sure thing!" said Spike, before turning and rushing down towards the kitchen.
After depositing her kite somewhere safe, Starlight used her ears to follow the sound of the argument, eventually coming to the outside of the room containing both the Harmony Thrones and the Cutie Map. She took one more deep breath, before lighting up her horn, and opening the door.
As expected, Twilight and Trixie were both inside. They had their muzzles pressed together so hard they were scrunched up, and their eyes were nearly touching each other. Both mares' horns sparked furiously with loose magic, and Twilight's wings were spread in an attempt to make her look bigger. From each of their mouths came a constant stream of enraged ranting, directed at each other.
"You think you're so tough!" Twilight snarled. "You're a second-rate magician with third-rate sleight-of-hoof tricks!"
"Oh, yeah?!" Trixie spat back. "How do you explain the Great and Powerful Trixie saving you from the changelings, then?!"
"If you're so Great and Powerful, how come you couldn't beat me in a magic duel without the Amulet?!"
"Pancake brain!"
"Oatmeal face!"
"Nerdy princess!"
"Stuck up pincushion!"
The argument dissolved into a slap-fest, with both mares incomprehensibly insulting each other while lightly smacking their hooves against the other's heads. Starlight had to admit, if it weren't two of her best friends, this would actually be sort of funny. However, she couldn't let it go on any longer. So she cast a quick voice-amplifying spell, took a deep breath, and screamed, "QUIET!"
Twilight and Trixie immediately lost interest in their little squabble, and clamped their hooves over their ears to drown out Starlight. After a few seconds, during which they realized there was no more noise coming, they slowly uncovered their ears, and discovered Starlight's presence in the room.
"Oh, uh...hey, Starlight," said Twilight, putting on a very awkward smile. Trixie had a similar expression on her face.
Starlight glared at both of them. "What did I ask you two to do the last time you had one of your little tiffs?" she demanded.
"Not start another fight. Or at least try to be civil with each other," responded Trixie.
"And what are you currently doing?"
"Having a fight," said Twilight. Both mares sank low where they stood, looking embarrassed.
Starlight slapped her forehead. "What am I going to do with you two? I respect both of you as friends, yet whenever you're within the same square mile, you get under each other's skin, and it's like a war is going to start!"
"I'm sorry, Starlight!" Trixie insisted. "But I swear, it wasn't the Great and Powerful Trixie's fault this time! She—"
"Do you seriously still need to refer to yourself as the Great and Powerful Trixie?" complained Twilight. "You're not on a stage, you know!"
"Oh, you'd love that, wouldn't you, you—"
"COOL IT!" Starlight screamed again, silencing them both. She sighed, before taking another deep breath to calm herself down. "Look, am I going to have to pry you two apart like a pair of foals whenever we do something together? Because I would really like to not do that!"
Trixie and Twilight both opened their mouths, likely to start blaming each other for the incident again, but Starlight's glare shut them up. They looked at each other, then at her, before looking down at the ground.
"I just don't understand it," said Starlight. "It doesn't make any sense. Why do you two even hate each other to begin with?"
"Trixie knows why she hates Twilight," responded the magician. "It's because she ruined Trixie's show when I was in Ponyville for the first time."
"But from what you told me, it was Rarity, Applejack, and Rainbow that interrupted your show," Starlight reminded her. "Twilight didn't do anything."
"Well..." Trixie stumbled over her words. "Twilight showed up Trixie by driving off the ursa major!"
"It was an ursa minor," Twilight added, quietly.
"Whatever! Trixie's point still stands!"
"Actually," Starlight mused, "shouldn't you have been angry at Snips and Snails for bringing the ursa into Ponyville in the first place? Twilight didn't do it to hurt you. What would you have had her do? Let it destroy the town so there weren't any witnesses?"
Trixie's eyes widened at the accusation. "Well, when you put it that way..."
Twilight took over. "I don't really hate Trixie, I just have trouble forgiving her after the whole Alicorn Amulet disaster! She tried to banish me from town!"
"You said that the amulet amplifies a pony's most negative traits," said Starlight. "Trixie, didn't you say you originally bought the amulet to try and revitalize your magician career, and you only thought of going after Twilight after you put it on?" Trixie nodded. "So, Twilight, how can you blame Trixie for something she did when she wasn't in control?"
Twilight opened her mouth, but closed it again a few seconds later. Evidently, she couldn't come up with a response. The look on Trixie's face told everyone that she was having a similar crisis. Starlight stepped forward, placing a hoof on each mare's shoulders.
"Your entire rivalry is built on false premises. There's literally no reason you two should be at each other's throats all the time. So why are you?"
The next few seconds passed in silence, as everypony's brain attempted to process what Starlight had put out.
"Trixie...I suppose I just liked the idea of having a rival," said the showmare, finally. "Twilight was just a convenient target."
"It was kind of exciting to have somepony to pit my magic against," said Twilight. "For what it's worth, I enjoyed our magic duels. They were a rare opportunity to use my brain as much as possible."
"There. Doesn't it feel better to admit that?" said Starlight.
Both mares nodded. Starlight breathed a sigh of relief. A knock at the door alerted her to the fact that Spike had arrived bearing the cool down snacks, and she telekinetically pulled it open. The drake came in, bearing a large plate with an assortment of food.
"You didn't specify what kind of snacks, so I got a little of everything," said Spike.
"Are those peanut butter crackers?" asked Trixie, perking up a little.
Sometime later, the three mares and dragon were sitting quietly around the Cutie Map, having finished off as much food as they could. Spike burped a few emerald embers.
"That was a good talk. Thanks for being there to help, Starlight," said Twilight. "You're the best friendship student ever."
"It's what I do," Starlight replied with a small grin, though she tried not to look too smug.
"I'm grateful to you for resolving our pointless rivalry," said Trixie. "I'm so glad you saved Trixie from making an ass of herself."
"Same here," added Twilight. "Though, I must admit, I looked far worse than you did. That whole fight was all my fault."
"No, don't be ridiculous," responded Trixie. "The Great and Powerful Trixie was responsible for the argument."
"Oh, no you don't! I'm the biggest jerk here! I'm the Princess of Friendship, for Celestia's sake! If I can't look past an old disagreement and just be friendly, what's Equestria coming to?"
"Trixie was the bigger fool, and you know it, Twilight Sparkle!" Trixie was shouting now.
"Um, girls?" Starlight tried to interrupt, but went ignored.
"I looked more like a fool than Snips and Snails!" Twilight snapped.
Starlight and Spike groaned as the two mares stood up in their chairs and continued yelling at each other.
"Here we go again..." complained Spike.