What causes addiction?

"Liking": Liking something whether it be a substance or activity to the point where there is a preoccupation resulting in harm.. Probably has to do with Dopamine in the brain which some people get more of a boost from than others. This is heavily genetically influenced. Some substances have such a profound effect that they tend to be more "addictive" than others-like smoked methamphetamine or nicotine.

Complex answer: Like most illnesses of the mind, addiction is the result of psychological factors, genetic/familial factors and social/environmental factors. The final common pathway of each addiction is through the reward system, which is a dopaminergic system, and it's possible that in those prone to addiction might have a dysfunctional system.