Final answer:
The nurse should explain that nicotine creates a feeling of calm by acting on the brain's pleasure system, yet it increases the risk of cardiovascular diseases by affecting blood pressure and causing arrhythmias.
Step-by-step explanation:
The nurse's best response to why a patient feels calm after smoking, despite the harmful and addictive nature of this habit, is that nicotine acts on the pleasure system in the brain, which includes modulation of dopamine and arousal pathways. This effect can lessen feelings of aggression and induce a sense of calm or pleasure. However, it's important to understand that nicotine also stimulates the sympathetic nervous system, causing an increase in blood pressure and potentially leading to arrhythmias and other cardiovascular risks.