Ever seen It's A Wonderful Life--specifically, the final scene? When you help others out of the goodness of your heart, you encourage those around you to do as much for you.
It actually is a "marketplace", of sorts--goodwill from you to them, for goodwill from them to you.
I do love It's a Wonderful Life, but it is hard to take this as a serious response to criticism or supporting evidence in defense of your opinion. You do realize that this is a nice Christmas movie and not a documentary, right? Should we be expected to take your stand as a serious one? Using the events in a sweet and gooey piece of fiction isn't too convincing a bit of support, although it is probably just a bit more relevant than the touching events in Rudolph or The Grinch that Stole Christmas.
Regarding the concept that, in particular, Christians are going to take care of all "others" in need because of a sense of "duty" or "love" or as a consequence of convictions to their own beliefs, the fact that so many fail to refrain from lying, stealing, treating their kids well, and staying faithful to their spouse (to be fair, folks other than Christians fail on these points, too) shows that this is not sufficient motivation to keep them doing what God and their loved ones would wish them to do. I have no doubt their is goodwill out there being put into good works and charity, but it is not something that can be counted on to assist all who may need help, IMO.