Your brain does it to ensure peak performance...it's called fight, flight or freeze. In this case it's less of a fear response, more of an anticipatory one. [i]Norepinephrine, Adrenaline and Cortisol [/i]are released into your nervous system to make you reactionary.
Basically, just go out there and do what you can do and enjoy it. It's why we do this sh*t dude.