I dont even think about it like that, I have a fair few now, and I know they are for life, but mine are just as much a part of me as the scars on my knees, arms, head...the only difference is these are nicer to look at.
I know why I have them, and that is to act as a constant reminder of something. The memory is often more important than the image. It's weird, I'm not young, so its not an impulsive thing, but the permanence doesn't really come into my mind.
Every time I take my shirt off and see my chest piece, I am reminded of good times with my old band, and that is really important, more so than the fact that I have stay true tattooed on my chest!