As they said last night in a TV show - the human body is the ideal canvas to paint and decorate.
I think the pain is worth it for the benefits afterwards. I hate pain, but I endure it during piercings because I know how much I will love the piercing afterwards. In order to get the tattoo or piercing you have to earn it. You earn it through pain.
Why "modify" the body? Because it's fun, because I can, and because I want to.