Indebtedness is a big one, and often very subtle. It's also a huge part of our culture and social fabric.
There's a reciprocity to gift-giving, that I think we all know at some level. If Aunt Sue always gives you expensive fine gifts, you feel obligated to give her more than just a trinket each year at Christmas. It gets complicated though, because there's normally an expression of personal value and emotion involved in gift giving. The "normal expectation is that a well-thought out and appropriate gift is an expression of how much the person values you, and how much they love you.
But it can be exploited by a canny game-player, who uses costly or dear gifts to create a feeling of obligation in the receiver. This is especially true when the gift recipient can't reasonably match the quality or cost of the gift in return. It leaves the recipient feeling that they are indebted to the giver.
Favors are similar, and even more devious. Favors imply that the recipient owes not an item, but a service. And since there's no expected exchange date, it's a future claim, an open ended claim. If the favor is asked of the giver, then the recipient knows that there will be a future claim, and in some sense consents to that obligation. Depending on the familiarity of the parties to the exchange, it can be no big deal. Most of the time, we trust that the favor asked in return will be equivalent in onus to that which was given.
But a clever person can exploit this substantially. A person can perform unasked for favors. For most people, even if you didn't ask for the favor, it still creates a sense of obligation and indebtedness to the favor giver. And an exploiter can perform a favor which is valuable and costly to the recipient, but not nearly as dear to the giver... then creating a large future claim. The purpose is clearly to create a future claim to exploit at their need. Often the purpose is to simply have someone "owe" them.
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Many people who dislike being recipients of charity probably dislike it for this reason - it creates a sense of indebtedness. Anonymous charity lessens that burden, because there is nobody to claim that debt. But it can still leave the recipient feeling that they're "account is overdrawn", in the cosmic sense.