Board Thread:General Discussion/@comment-26159109-20170417104929/@comment-25926288-20170417182720

The flower appears due to evil, dark magic wielding people. Regina summoned one, Black Fairy summoned a whole field. I'm guessing that in canon the other villains weren't that much more evil and magical than Regina. Or they died due to the random wintry weather.

Every kiss isn't TLK. There must be some sort of sacrifice or willingness to sacrifice involved, or going the extra mile. Regina broke the curse because "the extra mile thing" which includes the idea that she intended to break the curse. Thus Snow and David didn't break the Dark Curse. If David was somehow dying of pixie-dust-itis and there was no earthly cure and Snow kissed him during the curse with her memories intact, I'm certain that the curse would likely break. (Or Rumple specifically made the first curse to be Emma-exclusive.)