((Prompt: You're the best thing to happen to my life. I think I want to keep you around for the rest of it.))
Some time in the future, there will be a day.
Perhaps it will be a rainy one, like today. Perhaps it will be clear out. Perhaps the sun and moon and stars will have fallen out of the sky and left a hollow void above, I don't know. What I do know is that there is a day, and it is terrifyingly real, and on that day, you will be gone.
I will mourn that day. I will curse it with my every breath. That day will be known to all as the day Princess Celestia fell to despair.
I will weep rivers when I no longer have you. My tears will wear mountains down to nubs. They will hear my wailing in the Crystal Empire, and they will hear it in Saddle Arabia, and they will hear it across the sea in Zebrica.
I will never truly recover. I may one day smile again, but in the corner of my eyes there will be a forlorn shadow of grief. I may one day laugh again, but it will be hollow and joyless compared to how I laugh now. From that terrible day onward into eternity, I will carry a scar upon my heart.
There is nothing I can do to stop that day from coming. I have know countless ponies who have tried—wiser, cleverer, more powerful ponies than I. They never succeeded, no matter what price they were willing to pay. They only ever managed to make monsters out of themselves and their loved ones. They are the stuff of campfire tales and ghostly legends.
I cannot stop that day from coming. I can only pray for yet a little more time before it comes.
But our story is not a tragedy.
Ours is a story of joy and laughter, not of tears. Of a thousand soft kisses and whispered nothings. Ours is a story of lying in bed together, hooves wrapped around one another, tired from a long day, but too excited to go to sleep, telling one another every last one of our hopes and fears and dreams. Our story is of mad gallops across the park in the dead of night, tripping over roots and cursing and giggling. Our story is of warm afternoons curled up together in a spot of sun cast by the window, sharing content silence.
Our story is of our love, and no matter how the ending goes, it will never be a tragedy.
Because even if, when that day comes and you are not here, it breaks me completely—and it very well might—I was only ever whole to begin with because of you. I could weep until the end of time—and I very well might—and all those tears would be but a single droplet compared to the ocean of the tears of joy I have shed thanks to you. No amount of suffering or anguish could have any kind of meaning at all next to the bliss that you give me.
So please, smile for me. Do not worry too much about what the future may bring, because between you and me, I have already done more than enough of that for the both of us. Let me hear you laugh. Let me hear you sing. Let me tell you that I love you, and, if fate is generous, then let me hear you tell me the same.
Because when I say I will love you forever, it is not an idle platitude. I will love you this second, and the next second, and the next century, and the next millennium, long after my sun has burned itself to an ashen cinder and Equestria is nothing more than a distant, half-forgotten memory. I will love you forever.
That said, will you take this ring, and be my princess for a while?