User blog comment:FairyRaven/Fairy Tail Chapter 371: Tartarus Arc, Part 2: Song Of The Heavenly Dragon, Review and Commentary/@comment-7418318-20140215085929/@comment-1359322-20140215091424

I think Dragon Force activation is more than just simply having lots of the Dragon Slayer's element. Jellal's Flame of Rebuke clearly doesn't have a large quantity, the Etherion clearly isn't fire. Also if i based it on your theory, Wendy would've been able to enter Dragon Force in her match against Cherry since she did "eat" a large quantity of air in that match.