I would say it becomes a dependancy through mental constructs, not through chemical properties. 'Weed hangover' may very well be a symptom of non-cannabinoid products which get burned along with the plant when smoking. Effects reported include tired lungs, quite possibly a result of the method of ingestion.
Marijuana is not a gateway drug, except to the extent that it is a black market substance, and so gaining possession of it requires one to relate with people involved in other black market activities. This being the case, the smart choice is to avoid the situation entirely, as the benefits can be enjoyed through meditation without the possible legal side-effects.
Another reason for the gateway drug myth is that a person willing to try an illegal substance such as marijuana is a personality type more likely to try another illegal substance. Therefore studies which say 'X% of pot users have also used ___' do not lead logically to the conclusion that ingesting marijuana suddenly makes one more likely to ingest other drugs. The extent to which this is actually the case is a result of prohibition: 'Wow the DARE cops were wrong about this, what else were they wrong about.'
Looking down on users is common. The important thing if one wishes to judge is to separate usage from results. IOW, it is the proverbial laziness that is the problem, not the plant.
I find that marijuana smokers too often get involved with the rationalizations of other substance abusers. This causes too high a frequency of denial in the user. Alcoholics are notorious, but we also see pot users rationalizing heavy use and ignoring the axiom 'best in moderation' which certainly applies to pot use.
Society should not push simple substance abusers to the outside in the first place. Regardless of my personal beliefs on cannabis itself, the hypocrisy of the punishments is apparent when compared to the relative acceptance of alcoholism (It's beer thirty!) and even pain medication abuse. I recently got advice from a doctor which could easily be construed as 'I recommend this potential prescriptive addiction to solve your problem.'