My thinking on this has changed pretty significantly since I have had my pharmacology and neuro-psychopathology classes this year and last in med school. I've also heard several really world-class level physicians address this subject, one of them our dean at Southwestern, who's a Nobel Laureate for his groundbreaking g-protein research, which relates quite closely to cannabinoid receptors.
According to what they're teaching today in allopathic (regular M.D. training programs as opposed to osteopathic) medical schools, the cannabinoid, opiate and even nicotine receptor systems are very very closely intertwined in matters of pain, reward, and addiction. Those various drugs/substances act initially through their own receptors, but they eventually trigger a generalized reward system in the brain that can also be triggered by other drugs. So even if, in concept, cannabis isn't physically addictive, it can, in the right brain chemistry--most likely a brain that is genetically susceptible to addiction in the first place--trigger an addictive response. That's not behavioral, either. It's those opiate-cannabinoid-nicotine receptors and their reward system/response working together and becoming systemically responsive to the other substances. I believe this is why some people really do develop a dependency on cannabis, which I've read hundreds of examples here about. There are always plenty of "No, no way"-sayers on this subject, but the people who experience this are very definitely in the grips of something more than psychological dependence. Any one with a propensity for addiction can get addicted to anything, whether it has withdrawal-type dependence or not. Food. Shopping. Gambling. Opiates. Weed. Alcohol. They're those folks who're destined to get hooked to something no matter what. This is why, for them specifically, they truly are addicted, often to multiple substances, and why, for folks who fall into this category, cannabis easily can be a gateway drug. Argue all you want, but for some people it is addictive and it does lead to other types of drugs because of that response linkage to the other receptors. In these people, there is also fairly marked physical withdrawal when they discontinue cannabis after heavy and repeated use, probably as a result of the withdrawal effects on the opiate receptors.
Here's some good info, and please don't dismiss it. These researchers are learning more and more every day about why this happens to some people and not to others. This is seminal and significant research about these receptors, including several very good links and excellent bibliographical sources.
The Why Files | 4. Recognizing drug receptors
