Pony-itis!
Oh no! You've got pony-itis!
As you paced back and forth across your bedroom, you wondered how this could even be. How could you have missed the first sign? At least, you didn't think that it was unhealthy to have a desire to call a pony by a generic name as you hug them and pet them and squeeze them and pat them and scritch them and rub them and caress them and boop them...
Ahem.
But the cute, colorful, pony-eyed spots in your vision were proof of your condition... and your doom. Soon, you were going to disappear, never to be seen again. No one knew how or why it happened, and people had been tested and observed exhaustively, so all you could really do was try to tell as many family members and friends goodbye as you could, before you lost the chance to do so.
...And you would have, if you hadn't alienated everyone you knew and loved with all the fondness that you harbored toward ponies. So, instead, you stood in the middle of your bedroom and slowly looked around, taking in all of the pony stuff that you'd gotten over the years, in the form of plushies, dolls, figurines, toys, playsets, accessories, your bed's blanket, sheet and pillow case, throw pillows, clothing, curtains, wallpaper, posters, framed pictures, movies, comics, books, playing cards, writing utensils, stickers, rugs, slippers, dakimakura—
Oh, the eyes were rolling now. You saw darkness soon after.
...
...
...
Huh? Now you were looking up at a clear, blue sky while lying on the ground. You sat up and looked around, confusion quickly giving way to amazement as you gazed upon the cartoon village that surrounded you, one filled with... ponies! There were so many of them! While most of them were happily going about their business, some had stopped to regard you. They exchanged looks between each other, until they silently decided that the one in front of you should approach.
Was this a dream? Did you die and go to heaven? As the smiling mare approached, you shifted onto your knees, raised your hands and readied your fingers, waiting to strike. You'd soon have every pony loving you for your unique, bliss-delivering digits, and then all of your dreams would come true.
Once she was within reach, your hands shot out and your fingers went to work, one set between the ears and the other behind the head. Her smile remained, and she said nothing as you worked your magic. At least, you thought that was what you were doing, until you realized her expression was unchanged — rather than melting under your touch or twitching with delight — and that she had sat down on her haunches to patiently wait until you were done.
With a frown, you brought your hands back and looked over them, wondering what was wrong.
The pony cleared her throat, getting your attention as she casually informed you, "Sorry, dear, but fingers aren't all that special. We can play the piano, give massages, and pluck stringed instruments with our hooves, you know."
You were so devastated by that news that you recoiled, as if struck, and pressed a hand over your heart, feeling as if you were stabbed and about to bleed out. Desperate, you extended the index finger on your free hand and shakily delivered its tip to the pony's snout.
"B-boop?"
The pony's smile turned sympathetic. "I'm afraid that doesn't work either. Same with belly rubs."
You closed your eyes, fell upon your hands and hung your head upon hearing that, almost hitting the pony as she stepped out of the way. You must be in Hell, if you couldn't wrap ponies around your fingers. How else would you be able to get unlimited access to the adorable creatures and enjoy their company? And doing all those things to them that had once been restricted to the imagination, on account of them once being thought fictional?
You felt the pony place a comforting hoof upon your shoulder, although something didn't feel right about it. When you opened your eyes, you gasped in surprise when you saw legs ending with hooves in place of your arms and hands, causing your heart to race and inspiring you to push yourself onto your knees, so you could hold them out for inspection. Well, it was more like you reared back, due to having the hind legs of a pony as well, so you soon fell and had to scramble to land properly.
"Wha—!?" You began to voice, before turning your questioning gaze toward the pony, in order to address a more important matter. "How!?"
"That's the final symptom of pony-itis," the pony calmly informed you, before pointing to something over your shoulder. "There are mirrors set up over there if you wish to see yourself."
Looking in the indicated direction revealed a three-walled structure with the aforementioned mirrors housed inside of it, its general appearance and placement a bit reminiscent of a bus stop shelter. In your excitement you hurried over to it and entered its illuminated interior, not even thinking about the new limbs that carried you there, and began to inspect yourself from every angle.
Wow! Whether or not an alicorn was off-limits, you loved how you turned out and had no complaint about any of your new features. It's almost like pony-itis, whatever it was, knew exactly what traits you would have liked to possess for yourself as a pony. It brought a broad smile to your face, not unlike the one Pinkie Pie had expressed when she had seen the sonic rainboom for the first time.
Upon having the presence of mind to notice the other pony in the mirror, who appeared to be happy about your own happiness, you turned to them and enthusiastically inquired, "So, what now?" Then you blushed and added, "Oh, and what's your name?"
"Goodwill Seeker," said pony merrily replied, before sidling up beside you, slinging a foreleg over your withers and giving you a companionable squeeze. "And for the foreseeable future, you'll be staying at this halfway home until you can be integrated into society."
"R-really?" You asked, brimming with excitement. "I'll get to experience friendship and harmony as a pony in Equestria!?"
"Yup!" Goodwill Seeker confirmed.
You squeed and exclaimed, "This is going to be totally awesome!"
Unbeknownst to you, in an office later that day, Twilight Sparkle finished jotting down your name with a flair of satisfaction, adding it to a long and growing list of former humans.
With a sigh of content, she leaned back in her chair, glad that she was doing this as a "princess" of friendship instead of doing something stressful and extremely time-consuming, like worrying about matters of state, international affairs, running a school, or — Celestia forbid — doing any of those things at the same time. Thankfully, such irrational fears remained just that.
All she had to do was inspire someone on another world to create some "propaganda", and through that her self-made spell would find those with the right mindset and willingness to leave their world, transport them to Equestria, and transform them into ponies. It left her plenty of time to relax and enjoy herself, as well as solve the occasional friendship problem and stopping the latest evildoer trying to take over or destroy Equestria.
It was great, because she got to do all of the things that she loved: making lists, reading books, learning a bunch of new things, albeit mostly from another world, teaching new ponies about Equestrian history and society with specially-made books, organizing the support system and integration of said new ponies, and having plenty of time for friends.
The real beauty of this arrangement was that the new ponies wouldn't simply add more friendship to Equestria by themselves, but also pass it on to their children and rub off on the natives to some extent. It was far better than using her station — via obligation or loyalty — to spread friendship, inciting a need, or using some form of pressure, force or manipulation. She didn't even want to think about the state of a world that needed a school to teach any creature about friendship.
No, she was confident that a natural way would be better and more efficient: introduce those who already hold a strong conviction to the ways of friendship, then let them influence others and pass down their ways to the next generation. Nothing could possibly go wrong...