Supposing Applejack asks you out on a date. Maybe you had never thought of her like that before, even though she is a good friend. But it can't hurt to say yes and see how it goes. You're not opposed to the idea.
Supposing it does go well. She takes you to a cafe you both like on the edge of town. Because that is what is done on dates. And she may not know a whole lot about dating (neither do you), but tradition she very much does. Maybe she grumbles out loud that a good home cooked meal would be better, maybe she doesn't. You know she's thinking it. You walk through the countryside afterward and that is lovely. The sunset turns the sky orange and purple, just like you two.
Supposing she asks you for a second date. Then a third. Do you want to keep going? You look at your calendar; Tuesday evenings are free for quite some time. You chew the tip of your quill for a moment, then fill all of them in.
Supposing you sit with her under the trees one fine summer afternoon. You have a book of poetry and you take turns reading out loud. The poetry is very bad, but as she speaks the words disappear and all you hear is the love in her voice. Her laughter as she scoffs at the convoluted language afterwards is musical. You want to hold this afternoon in your memory forever.
Supposing that before you know it, it's your first anniversary. Your first, first anniversary. The past year seems so short, and yet not being together feels like a distant memory. Even in your dreams you walk among the apple trees now. The word "howdy" rolls off your tongue much more naturally than it used to. You don't exactly have a green hoof, but your little garden isn't doing poorly, and getting your hooves in dirt makes you think of her. You use the occasion to consider: you are in for the long term now. Exactly how long do you want that term to be? You are a serious couple now: exactly how serious? Now is the time to decide.
Supposing you fight. The details of what you are actually disagreeing about are quickly lost in the sheer butting of heads. You both learn how loud you can yell. You love her like you've never loved anypony else, and that somehow fuels getting mad at her like you've never gotten mad before. Eventually your friends get you to come around. But this will not be the last fight, you know. Can you take another one? Can she? It's ironic, there are times when you have admired her stubbornness, when she had stuck to her ideas and convictions for the sake of what she thought was right past when most ponies would have folded for mere convenience. On those occasions, when she comes out with her head held high in a rightful pride, she is truly beautiful. You could wax poetic about it. But here on the receiving end it is anything but beautiful. It's infuriating. It smacks even of ignorance. How can these be two sides of the same coin?
Supposing love seems terrifying now. It's tempting to run. What if you did and started dating Rarity or Rainbow Dash or anypony else? Assume, for the sake of simplicity, that this does not otherwise affect the dynamics of your group of friends. You would surely have fights and disagrements with them. You despair. Love was supposed to be so wonderful, and it was, and now it seems tainted. But you cannot be content being alone now either.
Supposing Applejack knocks on your door. She comes bearing a pie and an apology. Do you accept them? And is it possible that you were being a little overly dramatic yourself? You were not all wrong though, there will be more fights. But if you can get through them together it might just be worth it.
Supposing you see her every day. Once, getting together was an item in your schedule, now it is implicit, like brushing your teeth.
Supposing you take her home to meet your parents. You realize on the train that Canterlot will never be her city. She's attached to her land. If you stay with her, it cannot be your city any longer. Will you leave it behind forever? Dinner goes well. Your parents like her. Your mom coos over her accent and remarks how a little bit of it has crept into your voice. She gets on well with your brother. She tells you you're lucky to still have such nice parents. You hold her for a long time after that.
Supposing she takes you to her parents. Would they have liked you? Nopony can answer. She says they would have.
Supposing, one day, on your regular evening walk through the orchard, she asks you a question. The sky is your colors again, like that first date. You say yes. It shouldn't be as much of a shock as it is. You two have talked about this. This isn't a matter for surprises. All the same, you both are trembling.
Supposing you get married under the trees. Always the trees watch over you two. Mayor Mare asks "Do you, Twilight Sparkle, take this mare to be your lawfully wedded wife?" You look into Applejack's eyes. But it's not really a question, is it? It's a formality. The question was asked six months ago when you got engaged. If you were not going to say "I do" you would not be here, would not be wearing the dress, would not have licked so many envelopes and stuck on so many stamps and sent so many invitations. You kiss the mare with emerald eyes, and the warmest smile you have ever seen. Applejack who will stand by you no matter what, through nighttime eternal and twisted, laughing chaos, through all of Tartarus breaking loose.
Supposing your family is bigger now. That every time you think there can't be any more relatives a new clade pops up. All of them treat you like you had grown up with them. None pay the slightest heed to your being a unicorn—a Canterlot unicorn no less. Will you ever get clear of worrying about that?
Supposing your brother-in-law starts dating a unicorn too. It's not what anypony would have expected, but they're happy together. Her name is Sugar Belle and she seems to have had a rough past. You recall your dabbling with heroism and wonder if you could have done anything. The answer is no of course, she was far, far away from Ponyville. There's no possible way you could have known. There was no ready-made prophecy for her troubles.
Supposing Miss Cheerilee decides to move away. She offers you her place as school-teacher. You love your library as much as you ever have, but you do not live there anymore. Is it right for that bed upstairs to remain empty? And you will not be leaving it behind. It will still be there in the center of town. And think of the foals' faces, think of watching them grow and learn. Doesn't that thought make you a little giddy?
Supposing it's not everything you thought it would be. Many of the foals are much younger than you imagined. And even the most studious of the older ones is far short of the seriousness you brought to school. Not one of these students would be admitted to Celestia's School.
Supposing you had been one of these. Where in this town would you have found the drive? Nopony here has anything like your parent's collection of books. Your own lesson plans, overly optimistic as they are, would not have satisfied the filly you were. So suppose you were born here. Would that have... stunted you? It's a terrible word to use, but it is the honest one, the first thought that comes to your mind. But it wouldn't have mattered, would it? You would have gotten to exactly the same place you are now. A farmer's wife teaching in the school house.
Supposing, one day, as you walk back home to Sweet Apple Acres, you look up at Canterlot, perched high on the mountainside. It glitters in the low sun. It is not your home anymore. It hasn't been for a very long time. You're not even sure how much sense it makes to say that the same pony grew up there as stands in your shoes.
Supposing you don't miss it, exactly. You don't pass the hours on the farm longing to be back in Canterlot. What bothers you is that you can't go back, even if you wanted to. There is so much about that life that you really don't want to return to; you needed to come to Ponyville and learn the things you have learned. But there was a time you could have taken those lessons back there, done more, been more. What friends could you have made there? That time has passed. You are committed to this life now.
Supposing you feel bad for thinking any of this. To doubt Applejack at all is a betrayal. As your relationship with her progressed she became the center of stability in your life. Come what may, Applejack will always be there, and she will always be Applejack. It's about the heaviest-hoofed simile possible, but you have long though of her as a tree, big and deep-rooted. A tree standing atop a hill, beckoning you to come and sit under its shade. As immovable as a boulder, but alive.
Supposing you fantasize anyway. It is difficult, but your mind was made for difficult things. You do not marry Applejack, the love between you never blossoms. The time comes to part with your friends and return to Canterlot. You keep in touch with them, but you also make new friends, entwine your life with a new set of ponies. Meanwhile your studies continue; you are no longer a student but a scholar. What would that you be? You're talented and well connected, the limit is your ambition. You could be so much more than you are in any dimension. But would you be happier? Would you have love? It is easy to use the things you have now with certainty to ward off these thoughts of an uncertain alternative, but... What if you did have love? You would in any case have gone back much more ready for it than when you left.
Supposing you met a mare. Somepony more bookish than Applejack. Somepony with whom you could talk about magic, whispering words of spells and theorems to each other as you gazed up at the stars or lay together in bed... Yes, you could live that life and never regret a thing. What a horrifying thought. That life is closed to you. It is too late. Run away from this farm: too many years have passed, your youth has gone, your talents for high magic atrophied; that mare has a wife already, whom she does not question her love for.
Supposing you stop by the side of the road and cry.
Supposing you are waylaid there by Regret and Guilt. You have thrown so much away. Those dreams you had as a filly, those talents you spent so long cultivating have been wasted. You're teaching your class about fractions this week—when you were these foals' age you were teaching yourself calculus! And how ungrateful you are! To even consider throwing away your life, your family, for a few "if"s. Oh yes, that unicorn mare would be a lovely wife if you meet her, if she even exists, if you don't throw yourself back into a lonely pit of books and magic. At the rate you're going you deserve that loneliness.
Supposing you trot slowly the rest of the way. The thoughts won't leave you alone no matter how you try to dismiss them. You have done nothing; a mere thought is nothing to be ashamed of. It's only a fantasy, those are fine as long as you remember the real life you are living. Let the what ifs be what ifs.
Supposing Applejack comes out from the orchard to greet you. She asks you what's wrong. The tears have gone but they've left tracks.
Supposing you tell her—what? What do you tell her? Do you tell her that you've been thinking she's not good enough for you? Do you lie? Both are impossible. You want time alone to think, but thinking alone is the path that led you here.
Supposing you lean into her embrace, remaining silent. Have you ever felt as safe as this? "Welcome home," she says, and have you ever felt as truly welcomed?
Supposing the hot, quick love that you felt for her when you were young has faded, as it would have for anyone else. Deep down, there is still a glimmer of a poet's fancy that you could sustain yourself with only the sight of her smile and her shining eyes for food and drink. But this is not a fancy. She is here for you, she will be here for you tomorrow evening. You will share your bed tonight. You will brush each other's manes in the morning, and watch each other wake as you eat breakfast.
Supposing you tell her you were thinking about the choices you've made, about whether they were the right ones. Supposing you tell her they were.