Tattoos are generally obtained either for vanity or power. Vanity in that one believes that the tattoo they get makes them look cooler, tougher, edgier, etc. I once read an article on the power dynamic of tattoos. Something about how marking the body that god gave you lifts one's belief in their own power...a power assertion act, basically.
There are a ton of bad tattoos out there. I've even got a couple bad ones myself, but I don't begrudge anyone for doing what they want. As for the stupidity of some of the tattoos, some people just don't mature intellectually until later on in life. In my particular case, it was a conditional thing. As in, I was doing what Dave Attell calls "time traveling"...I only recall bits and pieces of the nights I got my ugly tattoos done.
If tattoos aren't your thing, don't get one, everyone's entitled to an opinion. But to b*tch about other people enjoying the freedom to do what they want with their bodies is...well, b*tchy.