An old favor can become controlling when it keeps being presented as a current debt.

Help that is mentioned once is a gift. Help that is mentioned repeatedly, at useful moments, has become something else, a standing account that can be drawn on whenever the other person wants something. The original act may have been sincere. What changes it is the repetition, which keeps you permanently in arrears and makes refusal feel like ingratitude rather than a choice you are allowed to make.

If someone keeps returning to what they once did for you, you can thank them for it warmly and still decline the current request. Gratitude and obligation are separable, whatever the framing suggests.