Warts are viruses that come out mainly in children, and the wide majority of children will never get them as adults because they developed an immunity to them.  

Patients get frustrated that the warts aren't cured with one treatment of liquid nitrogen. 

The reality is that warts live in many layers of the skin.  Each layer harboring the wart must be destroyed in order to keep the wart from growing back.  This takes several visits to the dermatologist before a cure is achieved. 

In addition, it is often helpful for the patient to use medication on warts like salicylic acid to allow more peeling of the skin layers in between office visits.