In the White's relationships, each decision they make--every cause they initiate--in regard to the monkey's paw has a direct result--a direct effect--that manifests in their relationships with each other. This shows that there is a correlation--a connection--between choices we make and how our relationships develop.
I am reminded of something a close person told me repeatedly--nothing's free. In this life, there is a cost to every decision. There is no such thing as getting something free. There is a cost to everything. The Whites learned the hard way that nothing is free. Mr White should have allowed the monkey's paw to burn.
The desire to gain financial happiness in "The Monkey's Paw" reminds me of many people's obsessions with lotteries. People buy lottery tickets in the hopes of getting rich quick and, usually, in doing so, they merely reduce their own savings. Like the paw's wishes, an unhappy outcome is the usual result.
The answer to this question is related to the fakir's desire to show the futility of trying to interfere in one's fate or destiny. It is the desire of the White family to try and meddle with their future that is directly shown to lead to the ensuing tragedy of the loss of their son.
This story follows the classic three wishes fairy tale. Bad things happen when the wishes are made. It's the rule of unintended consequences. You make a wish planning for something to happen, and then something happens that you didn't expect.
I would say that the major cause in this story is the White family's desire to manipulate their lives, to be in control of the forces that make the world work. They are not being particularly greedy or power-hungry, but they are trying to control things that they have no business controlling. So, in a way, I suppose you could call it hubris. Anyway, that's the cause. The effect is that "the world" or "fate" or whatever it is that they are trying to control ends up punishing them. It ends up twisting their desires around and making them into tragic consequences.