We are who we are.
Some people say that people that wilt get filtered out in the minors. I think some people just have so much more talent than other players that they will make the majors no matter what. However, once they get to the majors and their talent is equal to everyone else's, their nerves are more likely to play a major role. Also, some lights are burn brighter than others (think Melancon in Boston). Nerves may change depending on venue, importance of the game, etc.