In all honesty after 14 years of smoking weed I know these things to be true. I have not smoked every day of that 14 years and at some points didn't smoke one bowl for months. I currently smoke between about 0.5g to 1g per day on average.

After this many years I know:

1. Tobacco is really addictive - Holy Crap quitting was harsh
2. Weed does have a psychological addiction "profile" - I can 'feel' when I'm slipping into it.
3. Any effects of dependance are GONE after about 3 days

So basically I'm saying that I can feel when I'm slipping into a dependancy 'profile' where I am increasing my smoke intake, my tolerance is going up and I'm spending more time "musing" about weed in general. Sometimes it just plain 'bugs' me at the back of my mind and I begin to think of weed - smoking it mostly

However, after a few days of quitting (a bit like a detox) everything is back to normal again.

I would guess (a BIG guess) that this would be Seratonin/Dopamine related ~ it is definately along those lines for me anyway.

Each to his own, but that's my take and I think 14 years is long term enough to know about my body and the way it reacts to cannabis.