Why did the whispering increase in the "The Rocking-Horse Winner"?

The voices that rang throughout the house in D. H.
Lawrence's short story, "The Rocking-Horse Winner" always seemed to give the same, if
unspoken, message: "There must be more money! There must be more money!" Paul's desire
to help the family out financially drove him to ride his old rocking horse, and soon he
was able to amass a small fortune, which he decided to parcel out to his mother. But
when she decided she wanted the entire amount, Paul agreed to secretly advance her the
money. But the whispering only became louder.

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And yet the voices in the house, behind the
sprays of mimosa and almond blossom, and from under the piles of iridescent cushions,
simply trilled and screamed in a sort of ecstasy: "There must be
more money !  Oh-h-h; there must be more
money.  Oh, now, now-w !  Now-w-w --- there
must be more money ! --- more than ever
!   More than ever !
"

The more money Paul made,
the more money his mother spent, and the essential bills and necessities of the house
still grew. The whispers got louder and louder as more money was
spent.