Why do some people throw up easier than others?

I've always wondered this, specificly why my friends when sick always throw up all the time, but here I am having only thrown up once in the past 9-10 years (I am 18 at this point). Why do some people throw up every time they get sick and others rarely ever throw up at all? And its not that I'm healthier, I can tell you right now that i am far from a health junkie, I eat like a pig. I'm a healthy weight and theres no real physical health problems I face minus things like back pain and ibs, but the same goes for most of my friends. Is it because I dont have as sensitive of a gag reflex?