Rainbow Dash has been looking after Fluttershy ever since she was a filly; always there for her whenever a bully - be it pony or dragon - threatened her, and that relationship has survived well into adulthood.
Fluttershy has looked up to and respected Rainbow Dash for her irrepressible nature and confidence all this time.
One night, while stargazing together, they realise that there might be something more between them than simple friendship.