Assume a grossly consumerist, materialist view of the world, one in which we all agree that a pay increase makes us better off (other things being equal).
Now instead of getting a pay increase, every time you go shopping some stranger pops up and pays 10% of your bill. Are you better off? If not, why not?
Update: Before I forget that I had a point, this does actually happen on a large scale and people do moan. Where?