In most sports mental strength is the key to success. Take boxing for example. On the surface it may look like those guys are jsut punching each other in the faces - pretty dumb. But there is so much strategy in it, so much planning and prediction-making. You have to balance your calmness and agressiveness all the time because you can't lose your head and start charging to hard - you will lose energy too fast, and also you can't be too passive or your oppenent will take advantage of you.