Good question. Lots of factors, genetics alone can account for approx 50%, this is variable. Then the environment of that person, there upbringing, life trauma, other psychiatric problems such as bipolar. And yet this is not absolute i.e. Some people could have these risk factors and don't get addicted. Even more interesting is why do some addicted people get better and others don't, why do some seek treatment?
Addiction. Some people have a genetic predisposition to addiction, others develop addictions in an effort to cope with the stressors of everyday life. Often I have found people with addictions have great difficulty dealing with conflict constructively. My best.