The first thing that comes to mind is that people might be going out of their way in the name of gratitude.
Also the notion of gratitude or a perceived lack of gratitude is something that can be used as leverage.
Ie a person can do something for another which is not easy to repay or they might actually make themselves unavailable to be repayed in many ways.
I believe that some people might be more comfortable being owed more so than being payed back because the dynamic of the relationship changes quickly if they lose that lender's status.
It might even happen that a person will hassle one about a debt and when one attempts to repay they will forgive it right there & then.
Another circumstance is when one might do something for another and damn near use it against him and even if he has been repayed or if attempts at repayment have been made, the lender might not agree that he has been compensated in full.
Ive read that Buddha said something like; "Don't sacrifice your own welfare for the welfare of another, no matter how great. Clearly know your own welfare and be intent on the highest good".
Therefore it is crucial to be certain about what is owed and how it is to be repayed.
I've read in a Buddhist text something close to that one's parents aren't easy to repay and rather impossible to repay by means of carrying them on one's shoulder's. However if one ie was to establish a stingy mother in generosity then one more than repayed them.
I think this is a good example of having a strong constitution.
If one's parents ie say: "We don't want you to be meditator doing asceticism. You owe us. We don't want to listen to your preaching, who do you think you are thinking you will teach your parents and why don't you just marry a good girl, it would be good for you and we can have grandchildren.."
Another thing i recall from Buddhist studies is something like: "it is foolish to pick up a burden which isn't one's to carry and it is likewise foolish not to pick up a burden which is supposed to be picked up."
Another example of how gratitude might be used in manipulation is if a person goes to some general population prison and is housed with some big dude who shows kindness as he gifts items like chocolate, smokes & coffee and then one day asks for some sexual favors, saying something like: "i scratched your back, why don't you return the favor? You aren't ungreatful, are you?".