Physical Aspects of The Smoking Habit
At the center of the smoking problem is the physical addiction to the nicotine poison. Once you smoke for a while, your addiction to nicotine requires you to maintain a certain base level of nicotine in your body. Whenever your nicotine level falls below that base requirement, you begin craving another cigarette. That cigarette will fulfill the immediate requirement, but your baseline nicotine level will 'wear off' shortly, requiring more nicotine.
This is the vicious cycle of smoking. Your body craves nicotine, smoking a cigarette fulfills that craving, the nicotine "high" wears off, and you immediately crave yet another cigarette! The cycle is repeated as long as you continue smoking.
Other Physical Aspects
There are a few other physical aspects to smoking, also, such as the attention required by your lips. For each cigarette you smoke, your lips are accustomed to having that cigarette placed in there for a puff. At 10 puffs per cigarette for 20 cigarettes in a day, that's more about 200 times each day that you lips get attention.
This "lip attention" is one of the primary reasons why many people gain weight when they quit smoking! Since they are not smoking, they replace the cigarette-to-lip action with food-to-lip action and they overeat! It's really pretty simple.
The Tougher Issues
But, altogether, these physical aspects of smoking pale in proportion to all of the psychological and behavioral linkages and triggers that are also part of the complex smoking problem. What smoker has not experienced the need for a cigarette after food or caffeiene or an alcoholic drink? Or who doesn't need a cigarette when faced with stress?
To attempt to quit smoking for good without addressing these "non-physical" issues, will likely cause you much frustration, pain, and eventual relapse.