Twilight Sparkle is nothing if not a perfectionist; no matter what she does, she'll do it to the best of her abilities. When it comes to her relationship with Fluttershy, she'll settle for nothing less than the best. She has something important that she needs to tell her, and it needs to be said at precisely the right time. Because that's the sort of pony she is. It's just that ... it has to be at EXACTLY the right time with no margin for error.