Why are some drugs addictive?

Research have shown that all addictive drugs activates the release of dopamine from neurons in key areas of the brain. Dopamine is a neurotransmitter released by the brain. It plays several important functions and one notable function is that dopamine mediates pleasure. It is released during pleasurable experience such as food and sex. It can also be artificially induced by drug which can result in the user craving for more, leading to drug abuse. In some case, drug abusers may have genetic tendency that cause them to develop drug addiction even faster. 

Figure 1: What releases Dopamine

Why are drugs more addictive than natural rewards?

Some drugs releases up to 2 to 10 times the amount of dopamine that natural rewards such as eating and sex do. These drugs also result in immediate release of dopamine and the effect can last much longer than those produced by natural rewards. The powerful rewards of drugs strongly motivates users to take drugs again and again, resulting in drug abuse.