system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what is sea sickness?

What is sea sickness? Why do some people have it more commonly than others?
gpt
Motion sickness happens when your senses get mismatched signals. In this case, your inner ear tells you that you are moving but your eyes say you're still. Or for people who get sick from playing FPS games, vice versa. 

Anyway, the answer to why this makes some of us nauseous or sick lies in the question of when in nature might you get a sensory mismatch? Well, it can happen if you've eaten something poisonous and you'd be screwed if you didn't throw it back up. 

So if you had the psychological/behavioral reflex of what is now known as motion sickness, you'd have a chance at surviving after eating poisonous food. And this is a great adaptation for foragers like early humans. 

Why do some not have it? We're all simply different in our ancestral history and genetics and thus, we all have varying degrees of what life has to offer.