that doesn't mean every penalty miss is down to mental toughness. In some unique circumstances it may be. But generally you'd be good enough to hit that spot (say) 9 times out of 10 - you'll miss at some point, all the best athletes do.
Here's an interesting article about the psychology of penalty shootouts. I know we're not talking specifically about shootouts but a lot of the same stuff applies:
http://www.thesportinmind.com/articles/brazil-2014-penalty-shootouts-theyre-just-a-lottery-right/
Cultural traits make a difference too: