User blog comment:Miskos3/Fairy Tail 514: Dragon Seed, Review/@comment-1359322-20161212113105/@comment-7418318-20161212114031

It not only explains why they have motion sickness to begin with, but why it sets in only at a certain stage of their development in a way that makes sense. As they grew more attuned with the Dragon powers, their senses get sharper and the discrapancy between their Human and Dragon parts also widens.

I too like that. It sounds almost like that was planned from the beginning... or at least from the GMG onwards anyway, when it went away from a Natsu-only joke and toward pretty much a racial trait.