Question:
What makes this exchange outstanding--to me--is that we do have a great deal of case histories where brain damage incurred by accident or disease changed a person's emotional states and/or personality. Why is hard to believe that alcohol, which has been shown beyond doubt to change and damage the brain, doesn't cause similar changes?

we may be able to measure someone's P300 spike in their brain waves and tell if--and how much--they are at risk for alcoholism. Just "alcoholism"? Wouldn't the samething happen for other addictions as well?

Answer: They might do damage, but a different type of damage.

Brain damage has been shown to cause deteroriation of brain performance and changes in personality. So if these chemicals caused brain damage, they might indeed do some of the same things.

But--here is the kicker--do these chemicals stimulate the brain's motivational system, so the person is motivated to keep taking them?

Some chemicals are addictive (caffeine is somewhat addictive, for instance) but do little damage. Some chemicals (possibly the ones you listed) are damaging, but aren't addictive--they don't stimulate the patient to take more of them. Alcohol is actually one of the worst drugs, because it's both very damaging and (to those predisposed to the addiction) very addictive. It's that combination that makes it dangerous.