carbon monoxide preferentialy binds with hemoblobin, blocking it's ability to carry oxygen. It continues to bind and block the ability to carry oxygen long after you put out that cig. Only 2 cigs a day will greatly tie up the hemoglobin and cause shortness of breath. More than 2 a day...might as well smoke a pack!
Nicotine is the most addictive drug known. More than cocaine, more than herorin, oxycontin, more than anything. Withdrawal from cocaine and heroin may be more unpleasant, but the ability to quit nicotine, and stay quit, is harder.
Learned all this when I quit 1/1/1990. I still want one.