Why did Elya refuse to choose a number in order to win Myra's hand in marriage?

Elya refused to choose a number to win Myra's hand in marriage because he made a promise to his friend, Madame Zeroni, that he would carry her up a mountain to drink from a specific stream in exchange for her help. Elya believed breaking the promise would bring a curse upon his family, so he chose not to participate in the competition.