Moondancer has been in love with Twilight for a very long time. Even after Twilight didn't show up at her party, she still had hope that her feelings were reciprocated. That didn't happen.
Although they got off to an awkward start, Twilight and Pinkie have been dating for a few months now, and they are very happy together. The two ponies are very much in love, and although the news was a surprise to their friends, their relationship has no problems. But that's not what Moondancer wants. Not at all.