Tattoos, piercings and body modifications: why?

[Albertosaurus Rex]

These things are becoming increasingly popular and socially acceptable. Having a tattoo means you're hip. Personally, though... well, I'll be honest: I can't stand it and don't understand why anyone would do it.

Some time ago I was out eating with some fellow students, including a girl with a lot of tattoos. This prompted some conversation and what I really wanted to ask was simply "why?", but I was too afraid of sounding judgmental so I sidn't say anything. Hence me posing the question on the internet...