Ronan Thompson was a three-year-old boy with bright blue eyes who unfortunately did not make it to his fourth birthday. His mother, Maya Thompson, wrote a blog about him, detailing his life and his battle with a type of cancer called neuroblastoma. When Taylor Swift caught wind of it, she was inspired to write a song about him. According to Billboard, she contacted Ronan's mother first, and Swift even credited her as a co-writer on it. 

In the end, Swift wrote a beautiful ballad for Ronan, with heart-wrenching lyrics like: "Flowers pile up in the worst way, no one knows what to say/ About a beautiful boy who died." At the time, the pop star did not include the song on any of her albums. Instead, she used it for the charity event Stand Up 2 Cancer. Still, fans who did get to hear "Ronan" instantly fell in love, with some going as far as to name it one of Swift's best songs. 

So, when the singer-songwriter had the opportunity to re-record some of her past work, she clearly felt that "Ronan" deserved a place on the "Red" album era re-release.