As pretty much everyone here has already said, it can become an addiction, but it doesn't have to be. An addiction develops when anything, whether a substance, an activity, or whatever, is used to alleviate unpleasant emotions. The addict of course, in attempting to escape the problem (in this case through gaming), instead of dealing with it, doesn't solve the underlying problems, and sometimes makes them worse. In the case of gaming, an obvious example would be someone who feels lonely. In order to escape their loneliness they take up gaming. They spend so much time gaming that they become further socially detached, are more lonely, and in turn must play more games in order to escape the unpleasant feeling of loneliness. Especially if this cycle builds to the point at which the develop a sedentary lifestyle, their health begins to deteriorate, etc, then the addiction can become very serious, and eventually reach a point at which it would be nearly impossible for the addict to solve the original problem, which is that they are lonely, because they make themselves so unpleasant to be around.
Fortunately, that doesn't happen too often.