You can't stand to be bored. You like to play softball or volleyball with your buddies, but maybe they've become too predictable. Someone as competitive as you should pit yourself against the only person who can really present a challenge - you!
If you like to run, swim, and bike, you might be awesome in a triathlon. Or how about trying the Crossfit craze? You might really enjoy the hardcore style. And you can handle it better than other signs. Still, you don't have to prove anything to anyone but you.
And what sort of challenge would feed the Aries soul? The Ironman, of course! How many others can do that!?