People who have tried all the sensations that nicotine can give repeat nicotine using again and again. Even nicotine small dose effects can be strong enough to cause an addiction. And such a dependency is pretty strong as long as it is both psychological and physiological. Nicotine is an organic substance that is present in plants of nightshade family (like tobacco or tomatoes). Dry leaves of tobacco can contain up to 5% of nicotine. The synthesis and all the biotransformation of nicotine are performed on tobacco roots but it accumulates in leaves of the plant. Nicotine effects on human body lead to the use of tobacco leaves. In smaller concentrations, nicotine is a stimulant and these small doses of nicotine small cause a pleasurable sensation and are the main factors of habit-forming addiction. There are people that claim that it is not but in fact nicotine is an addictive drug.
All the nicotine effects can be biologically explained. It is the chemical structure of the substance that cases all the nicotine effects on human's body. Nicotine has a structure pretty similar to acetylcholine. Acetylcholine is so-called neurotransmitter that controls the release of more than 200 different neuro-chemicals in the brain. And nicotine being very close to acetylcholine can easily fit a host of chemical locks. So, nicotine effects are just the same as effects of acetylcholine. Once it gets into the body nicotine leads to a release of dopamine causing pleasurable sensation. Such a nice feeling makes a person want to repeat nicotine using again and again. That is how physiological dependency to the nicotine use is formed.
But there are other possibly even more dangerous nicotine effects that lead to a physiological addiction. As long as nicotine is a poison body attempt to reduce the dangerous nicotine effects. Doing so brain lowers the sensitivity to acetylcholine. All that changes the number of receptors sensitive to nicotine and acetylcholine. But the problem is that all these changes in the brain are based entirely on presence of nicotine so any attempt to stop using it will lead to unpleasant sensations and serious mood shifts. And that is how physiological dependency is formed.
But in most cases the nicotine effects are pretty short as they last only for about 40 minutes. That makes people smoke tobacco (or use any smokeless tobacco products) during the day to keep the dosage of nicotine on the necessary level. And that is also a problem. With time brain becomes tolerant to nicotine effects so the smoker needs to increase the actual dose of nicotine to reach the same level of sensations.
And because of such a strong dependency; it is pretty hard to quit smoking. Withdrawal symptoms of nicotine dependency include depression, anxiety and craving for dose of nicotine. And these symptoms will usually last for a month or even longer. For most of the smokers such experience is agonizing - that is why only 10 percent of people who try succeed in breaking their habit.
If you have decided to quit smoking the best way is to lower your daily dose of nicotine little by little. There are many different products on the modern market that will allow you to do that. Of course that won't allow you to avoid terrible symptoms of nicotine dependency withdrawal, but it is much easier to handle them that way.