Resentment
Resentment is the slow burn you feel toward people you keep saying yes to. It is often the bill that comes due for a boundary you never set, the buildup from giving more than you wanted to give.
You drove across town again, covered the shift again, listened for an hour again, and somewhere in there a low anger settled in. Often the anger points at them, sometimes at yourself. Underneath it is a simpler fact: you gave something you did not want to give, and no one knew, because you never said.
For people who please, resentment is the predictable tax on self-abandonment. Each unspoken no goes onto a tab. Because the boundary stayed silent, the other person had no way to honor it, and the cost lands as a grudge that can feel unfair to everyone involved.
Resentment is information. It marks the exact place a boundary belongs. The feeling tends to ease when the limit gets named out loud, even imperfectly, because then your yes can start meaning yes again.