With a physical addiction, the substance itself becomes necessary to the body. A psychological addiction involves becoming reliant on a perceived effect of some other stimulus. If you give a brain dead person a physically addictive substance and then stop, they will suffer withdrawals while they can't develop a psychological addiction as there is no perception.
Generally, the two will develop together and both are harmful.
There are a lot of people here speaking on behalf of smokers and people with chronic illnesses and disorders.
I happen to fall in both categories and self medicate.
I can and do stop smoking so the time. It is not physically addictive, and you can become mentally addicted to anything that gives you a dopamine release, like eating junk food.
I have never had "withdrawals," nor do I recognize weed "addiction" as anything other than excess, overindulgence, and a lack of self control.
People claiming that they are addicted to weed are simply looking to blame a bad habit for their lack of self control.
The only severe harm you can get from smoking too much weed is to your wallet.
There are many drugs that aren't strongly physically addictive, but most people recognise that the misuse of those drugs can cause severe harm. Cannabis should be in this category.