Twilight begins to learn more about her alicorn nature, and winds up discovering something about Rainbow Dash and Fluttershy that catches her off guard.
Rainbow Dash and Fluttershy have always been especially close. Few ponies can come up with a reasonable explanation for this. On the outside, it would be hard to imagine them being any more different.
However, on the inside, they couldn't be more alike.
They've always been the perfect match. Always.