This document discusses reasons why people smoke and the effects of nicotine. It notes that nicotine acts on nicotinic receptors in the brain to release neurotransmitters like dopamine and serotonin, which can make smoking pleasurable or reduce stress and appetite. However, smoking also leads to nicotine addiction as the brain requires a certain level of nicotine to avoid withdrawal symptoms. The document examines nicotine levels in the blood after smoking and withdrawal symptoms like cravings when levels drop. It notes smoking affects people differently and aims to understand smoking behaviors.