Out of all her friends, Sunset is closest to Pinkie and spends a good majority of her time with the girl. Partially because Sunset loves Pinkie a bit more than a friend. And apparently she's not that good at hiding it considering everyone knows. Given encouragement by Rarity to confess, Sunset says pretty much what the hell and goes for it.

If only Limestone didn't make Pinkie visit a choosing stone or whatever the hell Maud called it after the two have been dating for four months.