After harmony had been restored to Equestria after the destruction of Discord. Princess Celestia put herself in exhile, for being so foolish to fall for his lies.
Then while Princess Luna ruled the land, Princess Celestia used a spell on herself which erased all of her memories and turned her into a petite Earth pony-mare. The large amount of magic she previously had as an alicorn, was placed in the Tree of Harmony so that it could become even stronger than before and to make up for her taking the Elements of Harmony from it.
After she had gone through her transformation, her new self named: Seline Dijon, traveled to a small town outside of Equestria to start a new life. She managed to get an actual job at a café and a one-room apartment. One day though while she was working, she met a Pegasus stallion named: "Meteor Mash" who instantley fell in love with her. After further interactions, the two ponies fell in love with each other for real. With this moment right here, being the one moment where they ultimatley confessed to each other.
EDIT: I have lost the source, where I got the background from. So I am sorry that I am unable to credit the original uploader.