Prosperity: Why?

In the context of financial giving --

And God will generously provide all you need. Then you will always have everything you need and plenty left over to share with others.         2 Corinthians 9:8

For God is the one who provides seed for the farmer and then bread to eat. In the same way, he will provide and increase your resources and then produce a great harvest of generosity in you. - 2 Corinthians 9:10

Why?

We don't have to wonder --

Yes, you will be enriched in every way so that you can always be generous. And when we take your gifts to those who need them, they will thank God. - 2 Corinthians 9:11

Of course, I don’t mean your giving should make life easy for others and hard for yourselves. I only mean that there should be some equality. Right now you have plenty and can help those who are in need. Later, they will have plenty and can share with you when you need it. In this way, things will be equal. - 2 Corinthians 8:13-14 

God wants us to be the conduit through which He meets the needs of others for whom He cared enough to die.

“God is not glorified when we keep for ourselves (no matter how thankfully) what we ought to be using to alleviate the misery of unevangelized, uneducated, unmedicated, and unfed millions. The evidence that many professing Christians have been deceived by this doctrine is how little they give and how much they own. God has prospered them. And by an almost irresistible law of consumer culture (baptized by a doctrine of health, wealth, and prosperity) they have bought bigger (and more) houses, newer (and more) cars, fancier (and more) clothes, better (and more) meat, and all manner of trinkets and gadgets and containers and devices and equipment to make life more fun. They will object: Does not the Old Testament promise that God will prosper his people? Indeed! God increases our yield, so that by giving we can prove our yield is not our god. God does not prosper a man's business so that he can move from a Ford to a Cadillac. God prospers a business so that 17,000 unreached people can be reached with the gospel. He prospers the business so that 12 percent of the world's population can move a step back from the precipice of starvation.” ― John Piper, Desiring God: Meditations of a Christian Hedonist

 It constitutes a lost opportunity to give to what could count for eternity.  Remember this verse --

“Do to others whatever you would like them to do to you. This is the essence of all that is taught in the law and the prophets. - Matthew 7:12

If you and your children were hungry, what would you want prosperous Christians to do for you?

How different John Wesley sounded than today's prosperity preachers --

“Do you not know that God entrusted you with that money (all above what buys necessities for your families) to feed the hungry, to clothe the naked, to help the stranger, the widow, the fatherless; and, indeed, as far as it will go, to relieve the wants of all mankind? How can you, how dare you, defraud the Lord, by applying it to any other purpose?”