Addiction is an incredibly complex issue with numerous social and biological roots. There is no single cause or cure, and there certainly isn't any "immunity."

And yet some individuals, mice or men, can be exposed to "addictive" substances time and time again without displaying addictive behavior. People/mice in high stress situations OD with much greater frequency; think daily combat for people and barren metal cages for mice. Yet the exact same population, removed from the high stress situations fails to even become addicted. This turns the current theory of addiction on its' head. It should be noted that most people funding and conducting studies have monetary and political agendas. It is detrimental socially to addiction treatment to even study the effectiveness of 12-step programs, for examples, since the control population of addicts beat addiction at a faster rate. Dismissing evidence with a hand-waving "it's too complicated for you to understand" is the mark of someone who has something to gain(or to protect) by maintaining the status quo. If you understand addictive behavior as being a subdivision of self-destructive behavior and class it thusly it explains the ineffectiveness of current policy and treatment options.

As to the solubility remark, that's from here.

Replacing the hydrogen-bonding -OH groups with -OCOCH3 makes heroin much less soluble in water than morphine, but more soluble in non-polar solvents, like oils and fats. Therefore heroin has to be injected directly into the bloodstream, but once there it can pass rapidly through the blood-brain barrier which normally prevents the passage of water-soluble and large molecules. As a result it is much more potent than morphine, but its effect does not last as long. Again, once the heroin molecule is absorbed into the body, the acetyl groups are removed, reforming morphine.