As smoking is to the lungs, so is resentment to the soul; even one puff is bad for you.

The line uses a vivid health analogy to argue that resentment is not a harmless, private feeling but a self-poisoning habit. Like smoking, it can seem momentarily satisfying or justified, yet it accumulates damage internally—clouding judgment, narrowing empathy, and keeping a person tethered to past injury. The phrase “even one puff” stresses how quickly resentment can reassert itself and how vigilance is required: indulging it even briefly can restart the cycle of rumination and bitterness. The quote’s moral emphasis is practical rather than pious—urging emotional hygiene and self-protection by refusing to “inhale” grievance.