Although the term “Addiction” is used in the area of gaming, and also on this site, it needs some clarity.
Some interesting observations can be seen at the IGDA site (International Game Developers Association).

Passion – When you are passionate about something. It draws you to something; it increases the value of activities in your life; it increases your energy, your motivation, your creativity. You feel expanded, revitalized, and passionate.
Addiction – takes away from your life; it reduces your motivation to do things outside of the one activity. The hallmark of an addiction is that it takes away from your life. It makes you feel compulsively involved. You feel a compulsive desire to move towards that thing, where compulsion is being driven by some external thing rather than being driven by something internal; by your own internal creativity.

It is possible to differentiate between these 3 things:

1. Excessive use of something;
2. Dependence on something, behavior dependence, when you continue to do something despite external factors that tell you it’s wrong. “My wife keeps yelling at me to stop and I know it is bad but I don’t stop.” “My grades keep falling but I keep doing it.” “I’ve missed work 3 times this week but I’ve done it.”
3. Addiction to something, which is supposed to be physiological; cocaine is addictive because over time my body begins to crave cocaine whether or not mentally or psychologically I actually want the cocaine.

Things labeled as addictive should have the physiological component. The opinion is that this is where the whole idea of computers being addictive becomes problematic.

Gaming can fit the outwards characteristics of addiction, but is this physiological aspect also present?

The IGDA study agreed that games can be seen as addictive and cited a particular study done in 1999 that showed PET scans of people playing a gambling game. The scans showed increased levels of dopamine in the brains of the players, and based on earlier correlations made between increased dopamine and other forms of addiction, the study suggests that game playing is also addictive.  Of course, many activities can cause a similar effect, so gaming is not unique in this respect.

In the early 1950s, Peter Milner and James Olds conducted an experiment in which a rat had an electrode implanted in its brain, so the brain could be locally stimulated at any time. The rat was seated in a box, which contained a lever for food and water and a lever that would deliver a brief stimulus to the brain when stepped on. At the beginning the rat wandered about the box and stepped on the levers by accident, but before long it was pressing the lever for the brief stimulus repeatedly. This behavior is called electrical self-stimulation. Sometimes the rats would become so involved in pressing the lever that they would forget about food and water, stopping only after collapsing from exhaustion.

Sound familiar?