Resentment can follow from tolerating.
Where there is pain and suffering (physical or mental) but it is tolerated for the greater good.
I tolerate all kinds of shit from my kids, and I don’t resent them at all. I know that they need me to tolerate their foibles while they develop and grow. In other words the tolerating must be finite.
So it follows that resentment can come when tolerating starts to feel like it’s not finite. It’s potentially infinite and it’s the other person taking the piss!
“My gift of tolerating is being abused and I am a fool!” That sort of thing.
This could be resolved between adults by noticing it and through communication – telling the other when the resentment creeps in. This can allow the other to ask for a little more time, to argue there has been grow, and to reassure that there is no taking of the piss.