Our fatigue is often caused not by work, but by worry, frustration and resentment.

Carnegie’s line distinguishes physical exertion from the emotional and cognitive strain that often drains people more deeply than labor itself. “Worry, frustration and resentment” name three common forms of mental friction: anxious rumination about the future, the stress of blocked goals, and the corrosive replaying of perceived wrongs. The quote implies that managing one’s inner life—through perspective, problem-solving, forgiveness, or letting go—can restore energy as effectively as rest. It also reflects Carnegie’s broader self-help emphasis on practical psychology: fatigue is not only a matter of hours worked, but of how we interpret events and carry them internally.