ha. yeah good question, Fear just seems to creep up on you!
I remember the crazy things i used to do too, and when im at moto x i see these kids about 8 to 14 years old who have no fear and just go flying over the jumps. I guess thats the answer then, you develope fear when you no longer bounce! lol When i fall off my bike im rolling round on the floor for ten mins, when these kids do, they brush the dust off and try to do the same jump but at twice the speed to see if that works! LOL