Cutting can definatly become an addiction... I struggled with it for a long time and still do sometimes because I relied on it so much.
Theres kind of two kinds of addictions, physical ones are from smoking and nicotenne and stuff like that, then theres the psycological kind. You can get psycologically addicted to
anything. Even like. Skittles. So a lot of people get psycologically addicted to cutting because it is a release, and they learn to rely on it instead of fixing the problem at hand.
Also, cutting or hurting yourself releases endorphines and horomones and such, and that's the rush that people get addicted to a lot of the time.
As to why you are... I think that's a question you need to answer yourself.
I'd say it's probably because you learned to rely on it too much, so when you try to stop your body isn't used to it. Sometimes psycological addictions can manifest itself in physical ways. It did for me anyway... Sometimes I'd get naseous around a certain time period of not cutting, or cranky and irritable, sometimes it'd feel like my skin was burning and screaming out to get hurt or crawling around... But I think that is against just your mind playing tricks on you.
If you're trying to stop, one of the best things to do, just like with alcohol and drug addictions, is to find what is triggering you, and try to fix that. They're actually very similar I've found.
Hope this helped...