Answer:
Because they show their love for each other with the gifts and the narrator wants readers to understand that when someone loves someone, he/she will do anything in order for the other person to benefit. This is why the narrator thinks Jim and Della are wise.
Explanation:
The story had an ironic ending since Jim sold his watch to buy Della a comb and Della sold her hair to buy a chain for Jim's watch. This means both gifts are useless for both characters but it shows the characters' true love for each other and the fact that they would do anything for each other.