Lucidity
Maybe it's how the sun's almost always shining there, perfectly reflecting the happiness below. Or maybe it's the taste of magic upon the wind, the land literally flowing with the stuff. Maybe it's the bright emerald green of the thick carpets of grass spreading over the hills, dotted with little flowers in every colour imaginable. Or the clear waterfalls, sparkling like crystal in the morning sun and glowing like to moon itself at nighttime, unpolluted though it's been there for who knows how long.. Maybe it's even the kindness of the ponies.
Or maybe, just maybe, it's how simple everything is there.
Every problem can be solved with friendship, and the ones that can't are banished by the power of harmony. And even then they always get a second chance, eventually. Because there's one thing that doesn't happen here. Not by pony, not by elements, because it's not the answer.
There's never a death sentence.
He thinks that's what he likes the most of all. Perhaps that's why he always returns there.
The first time is a complete surprise, and he didn't figure out how it happened until it was all over. He soon learned that he's in a place called 'Equestria,' and it's primary residents are... Brightly coloured ponies. Which he himself was one of, although he was two shades of brown with the only bright colour being his blue eyes. His 'cutie mark,' as they called it, turned out to be a rather fitting hourglass.
His TARDIS didn't come with him when he goes. He's stuck right there, but he didn't mind. How could he? It was a wonderful place, with so much to learn.
He didn't meet anyone this time, and before he knew it he was back in the TARDIS, just after a regeneration.
How odd.
It seemed he went there whenever he regenerated, he noticed. He runs into two young ponies unlike any other he'd seen before. They had both wings and a horn. He approached them only to learn who and what they were, but ended up forming a friendship with them.
The next time he returned, They were princessess, the older's mane beginning to have pastel rainbow shades growing from the roots of her mane, blending nicely with the pale pink. The younger's mane began to resemble silver moonlight, but there was something darker in her eyes than last time.
A week into his visit, the three made a new friend. A strange little creature, the only of his kind. He called himself Discord, and though he was only as old as the princesses, he had much magical potential.
The last days of his visit flew by as he watched over his three friends, keeping note of every detail. He'd begun to enjoy these days, when he could take a short break from the troubles he had in his own universe.
He came back once more, only to find Discord gone and his little princesses all grown, with manes flowing with the magic of dawn and of the night sky. He didn't take long in finding out that Discord had betrayed them.
The princesses had done so much while he was away, defeated so many evil beings. Celestia told him all about their adventures as they lay on soft grass under a sparkling starry sky. Luna was quieter than she had ever been, and the Doctor had a feeling he wouldn't see her again for a long, long time.
He was absolutely right. Luna was gone, corrupted. Celestia was miserable. She didn't want to see him this time, locked away in her chambers. So, since he was always welcome in the castle, he spoke with the guards. They knew of him, Celestia was always talking about him, as Luna used to. The bat pony guards wanted to know all about space, and he was happy to share.
There was a new princess. Cadence. He didn't get along as well with her, but they were still considered friends. Princess Celestia refused to speak of her sister, for fear of bursting into tears. He understood this feeling very well. He wished them well, and went to see what had happened to Equestria in his absence.
He didn't stay long the next one. He didn't even visit Celestia. He stayed with an 'Apple Family,' who welcomed him with open hooves after he met the youngest, Granny Smith Apple. They were famous for their apple farm right in the town they founded, Ponyville. And he arrived right when they were in Zap apple season. He supposed he should have questioned how they were rainbow, but this was Equestria after all.
He spent ten days. He couldn't bring himself to leave sooner.
The next time, he was stunned to find out who was back. He went to the palace as soon as he heard, and was greeted warmly by Celestia and Luna, though the latter seemed a bit hesitant. He assured her that all was forgiven, and spent a week with them before he set off to Ponyville to see what became of Granny.
She was the last one remaining of who he'd known before, but he met her granddaughters and grandson, Applejack, Apple Bloom, and Macintosh. He didn't make it for rainbow jam this time, but he got to share wonderful stories with the family.
He left during the night after four day's time.
The next time, he met a young purple unicorn named Twilight Sparkle, who was supposedly a very important pony here. The protégé of his very own Celestia, of all ponies! Oh, she was brilliant. She could soak up knowledge like a sponge, so he gave her knowledge, and she in turn taught him about the Elements of Harmony.
He stayed for a month this time, before he had to go back. He wished his friend good luck and hoped that he would return before she too was gone. He thrived off of these visits. Yes, he drank in the peace as much as possible, and it kept him halfway sane in his world.
Turns out, a month was too long, and he'd slept for hours after the initial regeneration. Oops!
On the next trip- the one that would turn out to be his last, he met Derpy Hooves, a grey Pegasus pony with derped eyes. She might have been one of his very favorites. The most surprising thing in Ponyville upon his arrival was the gigantic crystal palace. He ran into Derpy and had asked her about it, and learned that Twilight lived there. Apparently she was some sort of princess now? He spent as much time as possible, for he knew he may very well never see her again.
He made sure he got back in proper time.
This time, when the burning glow of regeneration came, he didn't fade into Equestria. There were no bright colours. There were no ponies. It was a wide expanse of azure blue with sparkling white lights floating about. He looked down, wondering how he could stand. It was a glowing pathway, winding into the distance. The ground felt smooth, glassy. He couldn't find a single bump in it's surface. He breathed in. The whole place felt like a dream. A lucid dream. He took a few tentative steps down the path before setting off at a trot, hoping to find something more than the glow of star-like lights and blue. There was a disturbance after a while, the blobs of white converging into a glowing ellipse. It almost looked like the moon- He was interrupted when it exploded. He expected to be hit, but the stars passed right through him. It left behind a pony form, solidifying into.. Luna!
"What? Luna, what is this?" He asked, she must know, how else could she come here?
"What do you mean? You created it, after all. You brought me here."
"I did not! Why aren't I in Equestria?"
"You are. It's simple really. You were so depressed, Doctor. You're so lonely. You wanted friends who would never leave you. You wanted to feel alive." She paced about as she said this, avoiding eye contact. "It's.. It has always been staring you right in the face. I don't know why it's taken you so long to get to this point." She wouldn't answer the question directly, and it was making him nervous. What was she talking about? How did she know this? He left the worst of his story out, she couldn't... "The truth is, Doctor... Well, think about it. Had you ever heard of an Equestria, before you came here?"
He frowned. "No, but I don-"
"Does anything here seem realistic, at all? Even with your knowledge?" The Doctor was silent. "I shall take that as a no. Why do you come here only when you regenerate? Why is every problem so easy to solve? Why," She stopped momentarily, turned to look him in the eye, for the first time, "Oh why, does nopony question you when you say who you are? You aren't magic. They should be suspicious, don't you think?"
He opened his mouth to respond, then closed it, thinking. Every time he put another piece together, he got more and more upset. "That's not.. That can't be..."
She didn't seem nearly as upset, as if she knew the truth of her existence all along. "Why is this world exactly what you've always wished for?" She said, voice barely going above a whisper.
"No! No, No, No!" He panicked as the reality sunk in. Luna began to fade away. Then he was standing on the mountain he had first arrived here on, but it was different. The grass was dry, brown-tinged. The sky was turning dark, the sun blinking out of existence. He could hear running in the distance, Screams of absolute horror. Trees of the Everfree began to sink into the ground, quicker and quicker, until all that remained was the harmony tree. The world, his world, being destroyed only because it was his. Because that's what happened to everything he touched. He was the Doctor, and he destroyed worlds.
The harmony tree crumbled.
Just as he began to have a mental break down, tears soaking his muzzle...it stopped. The grass around his hooves turned greener, the brilliant colour spreading out from where he stood.... It all fell back to what he knew.
He woke up.