Mark Lewis spent 15 years hooked on just about any drug you can think of: alcohol, LSD, cocaine, heroin, opium, prescription painkillers.

Today, he is a neuroscientist and professor of applied psychology at the University of Toronto, who had just written a new memoir, entitled "Memoirs of an Addicted Brain."

But this is no drug addict tell-all. Instead, Lewis details honestly his life as an addict, while drawing on his knowledge of neuroscience and the brain's workings to try to explain it. Lewis says he wanted to explain why brains get addicted and says it seemed only natural to use his own experience to do so.

"Your moment-to-moment experience: your thoughts and your feelings are all paralleled by things happening in your brain, of course. And it seemed possible to weave the two together and that's what I was going for," he told CTV's Canada AM Wednesday.

Lewis believes the roots of his addiction trace back to his adolescence, when his parents sent him away for two years to a boarding school in Massachusetts. That's where he began drinking and experimenting with getting high off cough syrup.

But it was when he went away to university in Berkeley, California, that he got completely sucked in. That's when he began dabbling in psychedelic drugs and heroin.

He was able to finish his degree, but returned to Toronto with a BA in music and a serious drug problem. Even after he got accepted into graduate school to study psychology, he found he couldn't break his drug problem.

"The turning point came when I got busted and kicked out of school," he says.

Lewis says he tried many, many times along the way to quit. But, as he explains in his book, his brain had become hooked to the release of dopamine and the pleasurable feeling the drugs brought him.

"Addiction is really hard to understand," Lewis says. "That's why it's important to talk about the brain processes that are going on and that make it so difficult to stop."

Lewis says he really did have to hit rock bottom in order to break the cycle.

"It got bad enough, it got aversive and disgusting and horrible enough that I was able to stem the attraction and I was able to say, ‘I just don't want to do this," he says.

And to remind the reasonable side of himself why the addicted-brain side of himself had to stop, he put a sign up on his wall that read simply: No.

"I walked by 50 times a day and stuck to it," he says.

With all the research Lewis has done in his career – he's published over 35 papers in neuroscience and developmental psychology journals -– he still doesn't know whether it's nature or nurture that compels one to become an addict.

"It's some of each. But I think a lot of it is nurture," Lewis says, pointing to his own experience.

"Those two years I was in boarding school were pretty miserable. I was depressed. I lost all sense of who I was in the world. And moving from there to California, it was like, ‘Open up the pearly gates, I'm in heaven. There are drugs everywhere.' It was those series of events that helped propel me down that path," Lewis believes.

In this book, Lewis explains that it's our nature to crave what feels good and to seek it out again, even when we know it's killing us.

As Dr. Lewis notes in his book's introduction: "We are prone to a cycle of craving what we don't have, finding it, using it up or losing it, then craving it all the more. This cycle is the root of all addictions – addictions to drugs, sex, love, cigarettes, soap operas, wealth and wisdom itself."

Even now, 30 years later, Lewis doesn't feel he will ever be "cured" of addiction.

He recently endured a case of sciatica and was placed on the notorious painkiller oxycodone. After having back surgery, the pain began to get better. But by then, Lewis was taking two or three pills in the morning and then a handful more in the afternoon, convinced he wouldn't be able to handle the pain if he stopped.

He soon realized it wasn't the pain relief he loved about the pills; it was the warm feeling and the sense of calm they brought to his brain. He was able to quit the painkiller, but it drove home for him that he still liked drugs as much as he ever had.

"It was a reminder that addicts remain addicts. That's the old adage, right?" Lewis says.