Some people are tuned in in such a way that they don't worry about the pressure of competition - they play their normal game, no big deal. But a lot of people are more intense than that. And they can't come even close to playing like they normally do. There is so much to learn about competing, playing different courses - how to warm up, budget your time, or what have you. You're always going to run into some problems, and you need to gear up to conquer them.
People let their minds run away with themselves so much so that they never play good in tournaments.
The negative things just roll over them. Don't let that happen to you.