Nicotine, one of the most addictive substances you will ever know. Nicotine is the reason you continued to smoke after trying it a few times; nicotine is the reason why you have had trouble quitting in the past; and nicotine is the reason why you feel so poorly during the first days or weeks after you have quit.
There more natural methods that some people have done but I have to say I do not know how effective they are. Drink lots of water, keep yourself busy with exercise. Here are home remedies to quit smoking.