Here is what I posted when asked that question on this board a while ago. Some of it might not make sense without the context of the parent thread (clearly I am answering a few questions), but you get what you get from me now, I am feeling lazy:
The reasons are as many as there are people.
Seriously, there are many reasons. Some I agree with, some I think are silly. What matters is not what I think about people's reasons, but what they think of them.
The first time I saw Japanese full body suits I was fascinated by them. Not sure why, but I was. I like Japanese art in general. I have ZERO interest in getting pierced though, not that there is anything wrong with that.
Are they painful? Some more than others. On the meat it is mild scratching, on the bone it is intense pain, and on the tendons... YIKES!
Is it permanent, yes. Good. It is a commitment. Like marriage, or having children. Some people do so lightly, but those that do so thoughtfully are probably making a better choice.
How do I know I will like the work in years to come? I liked my first small one after a couple years, and it is simple, so this big one will probably hold its appeal. I also take ink well, nice color, good healing, and minimal fading. I thought about this big one for a couple of years, and it was about 4 year between seeing the print and starting the work. And it is art, art I will have for life, and enjoy the whole time. The more I looked at the print, the more I liked it, the more I saw in it. Plus my wife loves it. People who get serious ink are often called "collectors". That's what this is, a collected art work that I will enjoy as long as I live.
Some people get ink to be cool, to fit in, to try to stand out. Taz, tribal, barbed wire, lower back flowers for women, etc. Mine are not. They carry a great deal of meaning. Besides looking at them, they express ideas, emotions, meanings, and aesthetic sensibilities that I WANT to hold for the rest of my life. Some people get ink to remember a certain point in their life, to celebrate an acheivement, to remember a lost loved one. Mine is a mythical story, and that story resonates with my life in a variety of ways. It tells a story that holds a lesson, a lesson that it is good for me to contemplate with regularity.
Some people do it to feel a sense of control over their bodies, to make themselves into something, in their eyes, better.
Here are a couple links with info and stories:
http://www.bellaonline.com/articles/art27399.asp
http://www.everything2.com/index.pl?node_id=611881
The best way to understand why people get them is to read a variety of people talking about why they got them. The e2 link has some interesting stories for you to peruse.
I hope this helps. It is hard to answer the big "why" question, but I tried. The simple reason is... I wanted it!