Love is introduced in the first paragraph of the story as what the mother lacks. She is incapable, we are told, of loving her children or anyone else.
Money is introduced in the second paragraph as something there is not enough of in the family. The family is not without money: they live in a "pleasant" house with servants and they keep up a certain "style" of life that makes them feel superior to their neighbors, but the mother wants more.
In the story, love and money are linked. The incessant hunger for money—expressed by even the walls of the house seeming to say that there needs to be more money—becomes the way the mother tries to fill the empty space inside of her where love should be. Money becomes the substitute for love, but there is no amount of money that can compensate for a lack of love.
Paul feels the lack of maternal love and also internalizes his mother's need for more money. He believes that if he can get his mother the money she craves, he will, in return, earn the love he craves from her. When he finds out he can predict the winners of horse races by riding his rocking horse, he arranges to have his substantial winnings transferred to her, but they are not enough.
The incessant striving for love and money are symbolized in Paul's increasingly intense rocking on the rocking horse. What the boy is too young to realize is that no amount of money will ever fill his mother's empty spaces or earn her love. Love satisfies; money merely creates a dissatisfied thirst for more.