The adrenal system reacts to stress by releasing hormones that makes us alert and reactive. The problem is, the adrenal system can't tell what's a regular case of nerves and what's a real impending disaster.
The body doesn't know the difference between nerves and excitement, panic and doubt, the beginning, and the end. The body just tells you to get the h-ll out. Sometimes you ignore it. That's the reasonable thing to do. But sometimes you listen. You're supposed to trust your gut, right? When your body says run, run.