Why do women, and some men, pierce their nipples?

Asked by stevenb (3836) December 30th, 2010 from iPhone

I was texted a few pictures today, by a friend of mine who does that no matter how many times I say “no”, and it got me thinking. Why do people do that? What is it for? Why do people pierce like crazy anyway? Just a thought I have had for a while. Thanks!