Nicotine Addiction

There are three aspects to tobacco/nicotine addiction:

Smoking is a behavioural dependence. You may see tobacco as something that helps you in your day-to-day life, something that helps with stress or boredom. If you are young, you may feel it makes you look more adult.
There is a physical need for nicotine as well and smokers without even thinking will naturally adjust their nicotine intake to their normal daily average. That is why if you go on a long-haul flight you may find your pattern changing and you smoke more than you normally would immediately before getting on the plane and immediately on arriving. When the workplace became smoke-free you may have found that you had an extra cigarette in the morning and caught up on the others through the evening.
Smoking, rather than being a habit, is actually part of your daily rituals and routines, making patterns difficult to break when smoking stops.

Nicotine Addiction = How you think cigarettes help + your physical need + how your daily routines feel strange without the substance.

The nicotine ‘hit’ happens within 7 to 15 seconds of puffing on a cigarette when it reaches the brain. This fast action is quicker than most intravenous drugs.
This speed of action gives feelings of pleasure, arousal and reduced anxiety (although it may be that the anxiety is partly due to being addicted to nicotine in the first place).
In the short-term this increases the heart rate and blood pressure as nicotine is a stimulant, (despite its reputation for being a relaxant). People mistake this for an excited feeling, (a ‘high’).
Long-term smokers develop a tolerance to these effects and need to grow more nicotine receptors to maintain the same level of ‘high’ that they used to get, which is why you may start off as a three-a-day smoker and end up a 20-a-day smoker before you realise.