When you smoke, a spike of nicotine gets to your brain and reacts with a special area of the brain, where there are receptors that respond to nicotine. When these receptors are stimulated by nicotine from your smoking, they release chemicals that give you a feeling of pleasure. When you first started smoking, your brain only had a few receptors that reacted to nicotine. However, as you continued to smoke over the years, your brain has developed more and more receptors. Millions of them. Now, these receptors are used to that spike of nicotine.