User blog comment:IamJakuhoRaikoben/Chapter 441: Caracole Island, Review./@comment-86.70.179.52-20150622083824/@comment-39.42.80.105-20150622085413

Natsu had motion sickness before activating his Dragon Force for first time in tower of heaven arc.Gajeel haven't even activated yet and yet he still got motion sickness.

Probably have to do with Dragon Slayers reaching to a certain power level.