totally wrong on weed....
cannabis was previously thought not to be addictive due to the fact that it is fat-soluble and so the active drug and it's metabolites leave the body slowly and hence there is no comedown... also, it was previously not believed to influence dopamine. If a drug influences i.e. increases Dopamine or it's effects (by blocking dopamine uptake say) then that drug was considered to be addictive. Cannabis was found (within the last 10 years) to exert it's effects on a neurotransmitter called anandamide. Anandamide influences i.e. increases dopamine. Cannabis is addictive in some people.... why not all? Well, not everyone get's addicted to nicotine (another dopanine enhancer/increaser). While we're at it - steroids increase dopamine (via many pathways including the obvious testosterone increase).
The reason why cannabis DOES cause addiction in some people is largely due to a psychological addiction i.e. the user is self-medicating depression (dopamine and anandamide is effective for some depressions) and or the user is purely addicted to the euphoria (an un-natural state) of the dopamine/anandamide increase in their brain. Dopamine is the main neurotransmitter involved in addiction. But we have complex interaction here between anandamide and dopamine. Dopamine is no longer considered to be the only factor involveed in addiction - though it is the main one...
In conclusion, Cannabis is very addictive in some people, especially those with unresolved issues, depression/schizophrenia (cannabis is the most commonly abused drug by depressives and schizophrenics next to alcohol - probably again a result of the user attempting to alleviate albeit not entirely successfully the sympoms of the disease).