Addiction, by my definition, is the removal of choice. Choice may have been involved at some point but when the power of choice is removed you are left with an addict.
The number one question from anyone affected by an addict is, "Why won't he/she just stop". They see the consequences piling up and cannot comprehend how the addict can continue on their path.
The fact is, all of us are subject to our brain chemistry. If the brain demands it we will do it. Epileptics do not choose to have seizures, for example. As far as chemical dependency goes, I believe that different people have different thresholds prior to the onset of addiction. Some people are literally addicted from first use and others can go for years while still others never cross the line.
Once the line is crossed the brain is in demand mode. The fix will be had regardless of consequences. This is addiction and it is a physical manifestation of affliction that may be on many levels. With today's science addiction cannot be cured, only arrested.