Is there an explanation for Natsu's motion sickness? In Fairy Tail, Natsu is prone to getting sick whilst on modern transport. I am asking this question because it kind of annoys me when he gets motion sickness while fighting someone on board train or other forms of transport. These are the known forms of transport that he gets sick on such as...


Cars
Trains
Vehicles
... And many and all forms of transport that moves.

I think an exception to his motion sickness is when he is flying on Happy, but can't really think of anything else that he doesn't get motion sickness on.
And waves, just discovered in Episode 133 (quite strange).


Generally, he gets motion sickness from all non organic transportation. He also gets motion sickness when riding on other people. He is also known to get sick from looking or thinking about all different forms of transport, e.g. looking or thinking about trains. 

Was it ever explained in the anime or manga why Natsu suffered from motion sickness? And why did Wendy's motion sickness spell not work at all? Did something happen to Natsu in his life that made him suffer from motion sickness? If yes, please include it in the answer! Also, please refrain from using the reason "Natsu built up immunity to Wendy's motion sickness spell" in your answer, unless that's all you can possibly give me.

Answers are kindly appreciated.
Not only Natsu, but every wizard having Dragon Slayer Magic is prone to motion sickness. During the Grand Magic Tournament, all four Dragon Slayers suffered from motion sickness, i.e. Natsu, Gajeel, Rouge, and Sting. It is also confirmed in the anime that, Laxus suffers from motion sickness as well. The only exception is Wendy, but considering that she has Magic to counter motion sickness, it might not be affecting her.

It has not been explained, but it is likely due to a side effect of the Dragon Slayer Magic.

Wendy's Magic does not affect Natsu because he built up immunity. This was specifically stated by Wendy herself.

Also, Happy doesn't affect Natsu because Natsu doesn't consider Happy as a vehicle. This was also specifically mentioned by Natsu.

Edit as per the latest chapter: The motion sickness is caused due to inability of dragon slayer magic holder to adjust with the dragon's enhanced visual acuity which affects their semicircular canals.