Were we not in abundance the day before but now we are? What changed?
Also, bringing money into the game makes the responsibility stronger. Especially if those two persons are not living in a vacuum, but have other bills to pay [edit: i.e., to make other people do things for them]. Money is there to make people do things more reliably if there is no other relationship between the buyer and the seller.
“ Economic welfare cannot be adequately measured unless the personal distribution of income is known. And no income measurement undertakes to estimate the reverse side of income, that is, the intensity and unpleasantness of effort going into the earning of income. The welfare of a nation can, therefore, scarcely be inferred from a measurement of national income as defined above.”
This isn't the case with purchasing-power adjusted GDP, which is specifically adjusted to account for the actual purchasing power of people's money.