Twilight Sparkle has been lonely for a long time. Long before she moved to Ponyville she had discovered she was attracted to mares. The day she arrived in her new home, she laid eyes on the most gorgeous mare she had ever seen. A mare she was certain would never feel the same way about her. As time passed she fell deeper in love with Rarity even as she became more certain those feelings would never be requited by her friend.

Twilight x Rarity in case you hadn't guessed.