Co-Authored by my Beta: DragWiz91
Cadence is used to helping ponies with romance problems. So when she finds out that Discord and Celestia's past is basically an ancient love tragedy, she goes out of her way to give them a happy ending, or a new beginning at the least.
Unfortunately, or fortunately, this is the Alicorn of the Day and the Lord of Chaos. She probably should have realized they'd prank her for putting them in a pocket-dimension.