Transcending the physical
What is the appeal of body modification? Piercing, Tattoos, Scarification, or even Extreme Weird Stuff (CAUTION!!) ?
Some people like the way a tattoo or piercing looks. Some people like the shock value, or it is the "in thing" to do in their social group. Many people regret it later - opting for laser tattoo removal, or let their piercings close up.
For me it goes deeper.
It's my experience that life is a slippery thing - your circumstances change, your relationships change - "shit happens". It is very difficult to control, and often, the appearance of control is an illusion (*) . This can be very distressing for some people. Humans in general need to have a feeling of control ... over SOMETHING. The one thing I have COMPLETE control over is my body. So... it brings internal peace.
This is quite difficult to explain to people, and typically the reaction is something along the lines of "yeah right, you freak".
People will talk about a tattoo design calling to them. A particular location yearing to be marked. Often there is deliberation and then a sudden decision. The pain is a kind of right of passage which marks the event. It's very cathartic, and fun, and addictive. (**)
Another aspect I see that is appealing is the fascination over molding a living piece of flesh into an artistic creation. There are physical limitations, but not many. The presumption that the body we are born with should be left alone is ludicrous. Every day you get older, your body changes - take control of it. Make it what you want. It is the nobelest cause to take what is yours and remake it. It is... God's Will. (***)
The daily reminder is one of the best feelings. Looking down and thinking - "I did that. It wasn't there before, and now it is (grin)" Very cool (****).
So what do you think ? Where do YOU draw the line ? At some point do we just become freaks ? What are your personal experiences ? What is the first thought that pops into your head when you see something out-of-the-ordinary ?
And kids - be careful with that needle / knife / hot-poker / glue-gun
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(*) I once made the statement (paraphrase) - "You think you are free? - walk away from everything right now and see how free you are." Think about it.
(**) Interesting ironic note that the addiction is a loss of control. I'll leave it at that 
(***) In my opininion, cosmetic surgery, including breast enhancement, liposuction, etc, if done for the "right reason" can fall into this category. But there is an understandable suspicion that "mainstream modification" (my terms) are simply pandering to society's unrealistic standards of beauty. External vs Internal.
(****) The question "Won't you get tired of it,? Won't you regret it ?" completely misses the point. Do I get tired of my nose? Sometimes yes. Do I get tired of my feet ? Heck yea. If I had a third arm would I regret it ? Of course not. Why should the fact that you were born with something be any different than "adding" something.