This is a question that has been discussed over and over since the mid 1950s, and there has been some pretty good research that indicates that there is....or can be....a genetic predisposition to alcoholism, although I don't know if we can generalize that to all addictions.
It is essentially the old nature v. nurture discussion, and IMO the two are so interrelated that it's a losing proposition to even attempt to isolate one or the other. I am addicted to alcohol...and other drugs....and no one else in my family or extended family is. But they tend to be workaholics. Nature or nurture?
AA claims it's both physical and mental....allergy and obsession. I personally define addictions as an unhealthy, damaging obsessive dependence on any substance or process, and which causes signs and symptoms of withdrawal (physical and/or emotional) when stopped. This would include codependency which, IMO, is most often linked to a dependence on adrenaline (life via rescuing, crisis management and tilting at windmills). One reason codeps so often offload the alkie (usually far too late) and then find themselves another one. Dependency goes way beyond just a "habit," or even a lifestyle.
It is all insanity, and IMHO, nearly everyone has got some variety of it.
Which is why I love those in recovery....people who are getting well because they've come to understand they're sick.
blessings
zenbear