This article at Huffpo argues that addiction cannot be found as internal chemical hooks, but rather as a symptom of human boredom and isolation:
The rats with good lives didn’t like the drugged water. They mostly shunned it, consuming less than a quarter of the drugs the isolated rats used. None of them died. While all the rats who were alone and unhappy became heavy users, none of the rats who had a happy environment did.
At first, I thought this was merely a quirk of rats, until I discovered that there was — at the same time as the Rat Park experiment — a helpful human equivalent taking place. It was called the Vietnam War. Time magazine reported using heroin was “as common as chewing gum” among U.S. soldiers, and there is solid evidence to back this up: some 20 percent of U.S. soldiers had become addicted to heroin there, according to a study published in the Archives of General Psychiatry. Many people were understandably terrified; they believed a huge number of addicts were about the head home when the war ended.
But in fact some 95 percent of the addicted soldiers — according to the same study — simply stopped. Very few had rehab. They shifted from a terrifying cage back to a pleasant one, so didn’t want the drug any more.