I'm not sure about other/most people, but I was an extremely heavy smoker when I was younger. I quit at 17-18 (15yrs ago) and it was one of the hardest thing's I've ever gone through. I quit, because I started having Severe anxiety, paranoia and Constant, Severe brain fog which was bad enough... but when I quit all of them things got worse temporarily and I was hit with the most Severe depression I think a person could go through. It was HELL!!! Fortunately it didn't last forever though. It took a long time (years) for some of the symptoms to go away, but they did and I'm glad I haven't smoked since.
I know some people say that weed "isn't a drug" or that it's "not addictive", but it is (especially if you're a very heavy smoker for a long period of time). I believe that people not only become physically dependent on it, but that it is truly addictive as well (I just want to note that physical dependence is not the same as addiction. They are 2 completely different things).
On the other hand, I think the weed dealer I went to may have "spiked" the weed that I got from him. It was the cheapest weed on the planet, but it got me extremely high, even after my trip to Jamaica when no other weed would work. That could be an explanation to the severe withdrawals that I had. Chemicals (like Raid bug spray for example) can be extremely addictive and his weed did taste and smell like "dryer sheets".