It had seemed like a good idea at the time. The problem was, the last time something seemed like a good idea, it turned Sunset into a raging she-demon. And this idea involved Celestia.
The first time it happened, Sunset didn’t pay it any mind due to the circumstances. The second time it happened, she thought it was simply the action of a concerned friend. The third time it happened, she realized Rarity was flirting with her.