I think that the possibility that some people are "sensation seekers" (maybe have lower baseline levels of dopamine release in their basal ganglia when they engage in pleasurable activities) might explain a small amount of why some people get addicted to things, but it doesn't explain much.
I like what I read in the Lifering book ("Empowering Your Sober Self") about this - the basic idea is that any organism with a central nervous system can become addicted. All you have to do is repeatedly administer a euphorogenic drug to said organism and all of the symptoms of addiction appear. There's no organism (with a CNS, that is) that's immune to this. We're no different than any other person out there, we just crossed the rubicon for some reason and for the majority of us, we can't ever go back (at least, I can't).