Stress
Like any team sport, there's the stress of winning and losing. There's also the frustration of being close to winning but losing by one goal in the last second...and of being the reason that the entire team loses the championship in the last second because you were thinking of what you ate for breakfast that morning instead of concentrating on where the ball was.
But hey, it's a California born and bred sport. So chill, brah. It's only a game.