It's a scene Spike's seen many times in his fantasies, in his dreams. All their friends and family in dresses and suits, flowers everywhere, and a little white woven arch at the end of a long aisle. And at the end of that long aisle stands a beautiful bride in white.
It's Rarity's wedding day, perhaps the happiest day of her entire life. The least Spike can do is be happy for her. That's what a good friend would do, right?
A soliloquy.