People smoke tobacco because nicotine, an alkaloid found naturally in tobacco leaves, produces a soothing, relaxing feeling while also focusing the mind. In case you haven't heard, nicotine is extremely addictive. Nicotine releases neurochemicals such as dopamine into the brain's pleasure centers. Dopamine's purpose is to encourage survival instincts like eating, drinking water and having sex, so addicted smokers' brains crave nicotine like their lives depend on it.