Her selfless love for Jim and her wish to buy a Christmas present for Jim, because she was poor and the only thing she had to sell so she could buy him a present was her hair. So even though she loved her hair, she sold it to buy him the thing for his watch (I think) because she loved him even more than her hair.
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 <u><em>her wish to buy a Christmas present for Jim</em></u>. 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.
It effects them because everyone sees the world differently! Also, every single character has a different point of view and may agree or disagree on almost everything in any type of book!
