No matter what it may seem, or what is presented, or what you may read, please know this is a happy story. Rarity hopes you'll remember that. She'll try to remember, too.
The skies of the icy north aren't for the faint of heart. Even the best airship captain must be wary. But Blue Venture's not worried. She's sure Lunar Skysong will make it back to her in one piece. She always does.
If Twilight had learned something from her relationship with Rarity, it was that even if she was hopelessly in love with the other unicorn, she wanted Rarity to fulfill her dreams -- even if it meant sacrificing their relationship.
Life is like a train ride, don't you think, dearest? Destinations will change, trains will be caught and missed, but there will always be somepony willing to journey with you—so long as you let them.
For Twilight, the increased lifespan that came with princesshood had forced her to outlive her friends, but thanks to Rarity, she will learn that centuries could be lived in but a single day.