The addiction is not a psycological addiction but purely physical. What happens is your sinus cavity shrinks during the first 72 hours of use. During that time the sinus is swollen due to infection or other causes.

If you use it beyond the three day limit, your sinus' get's "addicted" in the sense that they will rebound or swell when the medicine wears off, not because of the infection that may have since passed. You then redose to stop that inflammation which inturn reinforces the cycle. It really is an easy addiction to get, in fact I'd guess it would happen more quickly then most other meds. So do not reach for it after the third day no matter how tempting it is, it will get worse.