Information about addiction in the Diagnostic and Statistical Manual of Mental Disorders 5th Edition focuses on substance use disorders and not on behaviors that have become habituated. However, the usereward cycle common to substance addiction can be present in behavioral habits although not as strongly “addicting” as substances.
The brain usereward cycle is explained by the National Institute on Drug Abuse as follows: “When some drugs of abuse are taken, they can release two to ten times the amount of dopamine (a brain chemical which is generically called an endorphin that produces a sense of pleasure or well-being) than natural rewards such as eating and sex do.” Sex, food, accomplishment, exercise, winning a competition, etc. also produce endorphins in the brain which encourage humans to engage in the behavior regularly. Because certain substances produce so much more dopamine in the brain, then humans learn (read that are rewarded) to take those substances often, even to seek them out. The increased dopamine can account for the fact that, in humans who are addicted to substances, basic human needs such as eating and sleeping and a sense of accomplishment can go by the wayside in favor of acquiring the substance.
The verdict is still out on whether or not humans can become addicted to behaviors such as exercise, sex, even frequenting social media websites like Twitter and Facebook. It is certain that engaging in such behaviors can become a habit. Generally, however, it is possible for humans to “break” a habit more easily than to stop using a substance that is physically addictive. Some research suggests that foods high in sugar content can work like a substance on the brain providing a strong usereward effect.
Finally, according to drugwarfacts.org, the top four most addictive substances known for humans at present are: nicotine, heroin, cocaine, and alcohol in that order.