The main chemical of the brain is dopamine. This neurotransmitter is responsible for many mental and physical processes. In fact, if you know how to stimulate the dopamine levels naturally, you will be able to overcome depression, anxiety, apathy, and fear, while at the same time you will boost feelings of pleasure.
Dopamine usually rewards some behaviors in us so we want to do them again. This is why some drugs are very addictive. For instance, cocaine, heroin, and other opiates can cause a reward of dopamine and this is why we like to try them again. Well, that is until you get a crash when the drug is absent from the physiology.
Opiates usually bind to the opiate receptors in the brain and these increase the release of dopamine. However, when this is gone, then there comes the need for more drugs to have the same high. This is why drug addicts usually would do anything to get their fix.
Studies on Dopamine
For instance, there have been some experiments done on mice and it was realized that when the same nerve bundle that usually causes an opiate release was stimulated when pressed a lever, the mice would press the lever thousands of times in an hour. This was all because of increased levels of dopamine. Then, after another experiment on human beings, they came to the similar conclusion. The person, after 3 hours, would press a button which triggered a dopamine dump thousands of times just to get an emotional boost.