There was a very good answer to that question, and Rarity knew it well. "Well," she started, "for one, jealousy is terrible for a friendship, or any sort of relationship, really. If you do know either of them, being jealous will only make things that much more difficult. It's even worse for romance - being jealous of an already established couple isn't going to make things any easier for you or for them. Although," and here she paused, thinking for a moment, "it is different if they're not already an established couple, but that still doesn't make it a good thing, mind you."

She paused again, giving her words a few moments to sink in and looking up at the nonpony. "The real question is, why would you be?"