If someone does you a favor are you obliged to do them a favor? Not necessarily. They may do you a favor to try and obligate you to do something for them that you wouldn't normally do, something that could be dangerous, wrong or even evil. However, if you receive a favor wouldn't it be a wonderful thing for you to pass that on to other people?
In one episode of the original TV series "Kung Fu" a young Kwai Chang Caine was granted a favor by a master. The master asked for no recompense nor would he accept any beyond explaining that he had done the favor for Caine because once someone had done him a favor and asked that he do a total of 10 favors for other people. Later in life Caine remembered that valuable experience and performed favors for others because of that one favor given him as a child. He asked for no favors in return. There was no silent debt created by these favors.
Imagine a wave of favors washing over the world and how much good will that could create. Imagine, also, how much good kharma you would accrue by these selfless, altruistic acts of kindness. Would not the world be a better place if we were to put this into practice? Try it - you'll be surprised how good it makes you feel.