There was once a group of white belts who practiced karate hour upon hours, day after day, month after month for many years. After many years of training, they decided to see who among all of them was the best.  They figured that whoever trained the hardest for the longest period of time, that person must be the best.  They noticed that one individual who had trained so hard, for so long, that his white belt had turned brown, and then black from all the sweat and hard training.  From that day on, it became a tradition not to wash your belt, but to let it symbolize how far you have come in training.