Tattoos and body Piercings for me are, as in ancient cultures, an initiatory experience.
My first piercing was my left earlobe, which my stepmother got for me at the age of 13 for my Confirmation (i immediately stopped going to church after that and had converted to paganism within two years)
The second was the Right earlobe, which I bought for myself after I turned 18. Partly to complete the set, and partly because I wanted to so something that I couldn't have done on by own before reaching that age.
Next was my left nipple, which I got the year I started college. I had a BF that had both of his done, and I wanted one of my own. It was also an expression of my "adult" life.
After that was my tattoo. I had wanted it for my 21st birthday, but it ended up taking till my 23 to settle on a design. It was bought for me by my best friend as a gift.
Just last december I finally Gradutaed college, and In celebration I got my Right nipple pierced. I thought it made a nice bookend to my college experience.
I have only one more body piercing that I want, and several more tattoos. But I am saving all of them for specific occasions. This year for my birthday I am thinking about getting another one done, and I know that when I have my body where I want it to be, I will get the bicep cuff I've been designing for the past few years.
I agree that the people who end up unhappy with their tattoos are usually people who got it as a fashion thing. I am happy with all of my body modifications except for the one that was forced on me (my circumcision). Each one has a personal meaning to me. I am hoping that the next one I get will be even more so, because I want to find a tattoo artist that will do the work in a more spiritual setting. Especially since the tattoo I will be getting next is tied to my spirituality.
I think how happy you are with your body depends quite a bit on the effort and thought you put into the work, just like with fitness...