Quitting smoking is easy. I've done it a thousand times.

That quote from Mark Twain in the title of this thread, made me laugh. However you realize why it is so hard to quit when you realize what keeps you smoking.

How nicotine hooks smokers

Nicotine produces pleasant feelings that make the smoker want to smoke more. It also acts as a kind of depressant by interfering with the flow of information between nerve cells. As the nervous system adapts to nicotine, smokers tend to increase the number of cigarettes they smoke. This, in turn, increases the amount of nicotine in the smoker's blood. After a while, the smoker develops a tolerance to the drug. Tolerance means that it takes more nicotine to get the same effect that the smoker used to get from smaller amounts. This leads to an increase in smoking over time. The smoker reaches a certain nicotine level and then keeps smoking to maintain this level of nicotine. In fact, nicotine inhaled in cigarette smoke reaches the brain faster than drugs that enter the body intravenously (IV).