User blog comment:IamJakuhoRaikoben/Chapter 461: That Perfume is for Somebody, Review./@comment-5894890-20151109061234/@comment-24546844-20151109114927

It's not just because Lucy discovered and mastered ir by herself that no other Mage should be able to learn it too. Lucy isn't egotistic like that. If Yukino asked Lucy to teach her she'd probably do it because it's in her character to do it. Even the spirits can teach it to her by themselves.

And it's not like Yukino can use it to it's full potential with only three golden keys.