Why do I crave cigarettes?
Someone who has been smoking for a significant period of time, meaning months to a year, has set up pathways in their brain where nicotine releases chemicals that feel great, and then those chemicals drop and you don't feel so good. So you smoke again and you get another high and then another drop. That pattern of nicotine highs and then withdrawal, create the sensation that smokers begin to associate with "I'm afraid I'm addicted". That pathway has a physiologic mechanism that will call for nicotine again.