There is this thing called physical dependence. Your body physically becomes addicted. Physically. And besides if that’s your explanation everyone has mental issues
Nicotine withdrawal symptoms aren't that extreme though (I've been there). And yes, more people than you imagine have mental issues, not necessarily of such character that they have high life-altering consequences. Humans are strong and small mental issues mostly go unnoticed because they aren't evident unless you're a careful observer.