Stress
The score is tied, the game is in overtime, and it's your turn to take the penalty kick. No pressure. The only things on the line here are your reputation, your team's reputation, your chance at a salary raise, your chance to renew your contract, and possibly your own life. Famously, Andrés Escobar was murdered after scoring an own goal against the US team in the World Cup in 1994.
But really, don't sweat—you're sweating! Why are you sweating? Oh yeah, it's over ninety degrees out here and you've been running around in the heat for the past ninety minutes. Just try not to sweat into your eyes, huh? You need those if you're going to make this goal. And you have to make this goal.