New scientific research is finding there is something about repeated drug use that changes the brain at a deeper, more permanent level.
Excerpt:
"Repeated drug use leads to long-term changes to the brain. Some of those changes, new research suggests, might be hard to reverse and might even intensify right after withdrawal, explaining why it is so hard to stay off drugs."
https://www.statnews.com/2017/04/19/opioid-addiction-relapse-science/