The "Selfish" Drugs
Both cocaine and nicotine cause intense pharmacological effects in the brain, generally related to the release of dopamine. Empathogens, found in other social (and illegal) drugs such as ecstasy, trigger the release of far more serotonin than dopamine. The higher levels of serotonin will typically promote empathy, trust, compassionate love and sociability. Generally, dopaminergic drugs, if taken on their own and to excess, can easily have the reverse effect, i.e., users are likely eventually to alienate family and friends. They tend to become isolated and suspicious. Simplistically, dopamine-based drugs, cocaine and nicotine, tend to be "selfish" drugs.
In the case of crack addicts, we consider users "addicted" as most of their money and time is spent thinking about how to get more of the drug. For any drug to be truly addictive, it must cause the user to demand it in a compulsive way. Need or desire for the drug makes the user lose control of the amount of time they take or the regularity with which they take it. When a person smokes a cigarette, they take an average of about ten puffs. Each puff gives them a nicotine hit, and throughout the course of the day, an average pack-a-day smoker will experience about 200 hits. Taking all these hits every day adds up. There are numerous studies that have documented the loss of revenue as smokers feed their addiction. This costs the nation millions of dollars in lost productivity each year.
Think of the people we all see each day huddled in or around a sheltered doorway, battling the sometimes bitter cold and harsh elements, just to get a puff of nicotine. Puffing and getting a pay-off in the form of a "brain-reward" so often, each and every day, sets up a whole series of associations that can be extremely difficult to break when the smoker wants to quit. Essentially, everything the smoker does becomes associated with smoking, i.e., drinking, eating, talking on the telephone, driving in car, etc. Smokers develop a whole host of behaviors that are connected with smoking. The need for a nicotine fix begins to dictate who they will associate with, as they seek out other smokers who will understand their need to keep on puffing.
Source: http://tobaccofreedom.org/issues/addiction/