No one who ever gets a tattoo seems satisfied; so they get another, and another, and...
I knew I guy who only got one tattoo while he was in Germany.
Blue numbers on his forearm...
It's the "broken window" syndrome.
Once one window gets broken on an abandoned house the rest quickly follow. After all, the house is already ruined.
Once the skin is defaced may as well get another tattoo as the perfection of the skin cannot be restored.
Works the same for virginity. Once a girl loses her virginity she tends to be much more willing to screw the next guy (and the next, and the next etc). Why restrain herself when the damage is already done?