Studies on drug addiction in 1998 showed that marijuana is as potentially habit-forming as the 'hard' stimulating drugs, since the habit-forming mechanism is the same, thus facilitating the step from marijuana to the others. I shall attempt to synthesize from what I read:

1.- Molecules produced in certain brain neurons determine the unpleasant feeling of anxiety.
2.- Other brain neurons produce anxiety-preventing and relieving molecules.
3.- Habit-forming drugs --abused for 'highs'-- inhibit for long periods of time the production of the anti-anxiety biological products . The habit-forming derives from the fact that the defensive brain products can not now prevent anxiety.
Pharmaceutical molecules capable of tiding over the anxiety solve the acute need for anxiety-relieving --habit-forming-- drugs of abuse.
What about patients that relapse?
Their situation may be due to some genetic brain chemical imbalance, or to not yet detectable anatomical brain defects. Psychological determinants may play a minor or major role. It is essential to make the differential diagnosis.