Masochist

Masochism is 'a sexual fetish characterized by pleasure in being subjected to pain or humiliation'.

The origins of western piercing, when not tied to the SideShow, tend to have a close connection with the BDSM (Bondage, Discipline, Sadism, Masochism) community. Aggressive bottoms found that their desire for pain was well met with piercing, particularly "play piercing" - temporary piercings done for the sake of sensation.

The word itself comes from a man named Leopold Von SacherMasoch. Known for his love of pain, his life and exploits were chronicled in the classic "Venus in Furs".

It must be noted that most modern body mod fans would say that masochism had nothing to do with their decision to get pierced/tattooed/etc.