Answer: her wish to buy a Christmas present for Jim.
Explanation: In O. Henry's "The Gift of the Magi", despite loving her long hair, Della cuts it and sell it. What motivated her to take this step is her wish to buy a Christmas present for Jim. Della is proud of her hair that reaches below her knee. It is like an ornament for her. It's Christmas Eve, and in order to buy a present for Jim, Della sells her hair. With the money she gets, she buys a golden chain for his watch.