In The Bridge to Terebithia, why does Jess have to be the fastest runner?

Although I am not a teacher myself, I would have to disagree with rmccord. I would do this because she said that jesse was the fastest runner in the 3rd, 4th, and 5th grades. this is only somewhat true. He only won the races onece in his entire life, and if you ask me, that does not make him the fastest of the 3rd, 4th, and 5th grades at all.