Gambling, drinking, taking drugs and overeating are all considered forms of addiction. Some believe that shopping can be addictive too. The high that is attained through buying something special can suck you in and make you want more.
When it becomes a real problem is when you can't afford your habit anymore and your closet can't accommodate your habit anymore. Just look at Katie Holmes, Jessica Simpson or Lindsay Lohan. Do you think they have a shopping addiction or just a love of the sport?