Rarity can see it clearly.
The way Applejack laughs around Coloratura, the way she smiles. The way Coloratura hangs on Applejack's every word, the way her eyes light up whenever they glance at each other.
Friends since fillyhood, Applejack and Coloratura share a relationship deeper and more special than most ponies will ever know.
But Rarity and Applejack were fillyhood friends too. Shouldn't that be just as special?