Throughout their friendship, Fluttershy wrote letters to Rainbow without the intention of giving them. This time, she's writing her last one and finally gifting all of them. It'll be her goodbye present for her.
Fluttershy has been emotionally broken by her ex-coltfirend and her friends want to get her back to normal, by fixing each problem one by one. Rainbow Dash also decides to tell Fluttershy about her feeling for her.
The darkest habit one could allow their self to indulge in is the one they know they can not live without, for Rainbow Dash, she should have just learned to free herself from the shackles that bound her from her former lover.