Addiction to dissociatives is much more common and plausible than addiction to psychedelics (which also happens to some people). Dissociatives, especially MXE, do not exact the same toll, at least at first, as psychedelics. A person can do MXE, feel awesome, and have no hangover or side effects to speak of. It's very easy to rationalize doing it again soon, and then again, and again, with increasing frequency. It boils down to the individual. Some people don't have particularly addictive tendencies, and some do. For someone who has them and for whom MXE fulfills something they're looking for, addiction is quite plausible. Whereas for psychedelics like LSD, the experience is very intense and people are not usually inclined to do them again right away.
Addiction can happen with anything. People get addicted to food, sex, and all sorts of more bizarre things (ever watched "My Strange Addiction"?)