Another great question:
My first thought was — “he/she keeps bringing it up” or “they’ll keep reminding me what they did for me”–
but then I realized there is a phrase we often use to describe this behavior: to hold something over someone’s head.
Here is the definition from our app (Check them out here on iOS App Store and Google Play!):
Although the person who did the favor may not try to “control” his/her counterpart, holding it over their head implies that they keep reminding the person what they have done for them. The phrase is used in multiple contexts, not only for blackmail but also in speaking about providing favors.
If a person can “hold something over your head” it means that they can taunt you about something–perhaps to scare you or bother you. It’s like when you dangle a toy over a child’s head–the kid reaches for the toy, but you keep it out of reach. It’s like you are showing the child what is there, but he/she can never have it.
If a person has some information that they “hold over your head”–they remind you that they know something you did (something you may be embarrassed about, feel guilty about, etc.) or they remind you that they did something for you (reminding you that they went out of their way to help, make you look good, etc.). It doesn’t feel good when they do it–and you hope that they forget it or stop bringing it up.
Example: One time I was tired and my friend offered to give me a ride home. The thing is, she kept holding it over my head–every time I saw her, she talked about the time she drove me home. It was her way of reminding me what a great friend she is. Go figure!
Example 2: This guy from work helped me fix my car — I offered to pay him, but he said “nah, I think I’ll just hold it over your head at work.”
Hope this helps! 🙂