I think it's addictive because of the natural dopamines our brains release to counter the pain of the fast needle stabbing your skin over n over n over n ov... Not totally sure though.
I love when my artist tells me stories of all the big burl "mans man" dudes who come in sayin it won't hurt and al that and then when he starts the tat, they pass out. Seriously, those stories don't get old. He said he had a marine pass out on him and then at a show he watched an ex navy seal pass out.