Also, it has been theorized that humans are hardwired for addiction, as it serves as an evolutionary advantage. We are made to pay special attention to salience, or special relevance. Drugs have the ability to take advantage of this "special programming" by heightening our decision making skills, conditioning, memory, thus sending the aforementioned salience into high gear. This, my friends, is what causes the craving. And although some people are more susceptible to addiction, if anyone is exposed to an addictive substance for too long, genes won't matter anymore. At that point, the other side of addiction, failure to cope, is to blame.