It's designed to suit Selphie's personality.(IMO)

Selphie is a carefree person that likes to do whatever she wants. Therefore, when reading a story book, one would presume that she would always want to see a happy ending or she'll end up tearing the book to pieces before saying "I HATE SAD ENDINGS!! GIMME A HAPPY ENDING!!"

And, the sea of flowers can actually bring out the feel as though you are in a fairy tale. It was a happy ending, like "Everyone lived happily ever after. THE END."

And there you have it, THE END. That means ending something. So, even Omega will bow to a happy ending. Of course this is just my theory.