The narrator says it best himself when he refers to them as "young fools", in the way that they are innocent, young, and naive when it comes to prioritizing, and to live in a world outside of their immediate small family circle. In a way you can also say that they are dedicated, committed, and innocent as well because they have shown each other a strong form of devotion in the sacrifices that they had to endure for each other's sake, but at the same time are still too fresh into the relationship to worry about the sacrifice being so big.
Three words that could describe the relationship between Jim and Della are "deep," "unselfish," and "reciprocal." The relationship between them is deep; it is not based on trivial or inconsequential surface qualities like a crush or infatuation would be. Della does not love Jim any less for having had his wages cut by one third since she married him, and Jim does not love Della less when she cuts her hair, although her hair was stunning. They love each other for the person underneath all the outward trappings.
Their relationship is unselfish, which is the point of the story. They love each other so much that each sacrifices the possession most valuable to him or her in order to purchase a fine gift for the other.
And their relationship is reciprocal; Della loves Jim as much as he loves her, and vice versa. Their gift-giving shows the reciprocity of their love as well as its unselfishness. Jim's watch is of equal worth to him as Della's hair is to her, and each of them knows and values the appreciation the other one has for his or her prize possession. So their choice of gifts is not actually surprising, but in fact is the most predictable of outcomes given the complete reciprocity of their love.