This is just my opinion.
There's 'art', which is subjective anyway so this statement may be a generalization but fuck it, and then there's 'wanting attention'. A lot of these people likely fall between those two lines, but I imagine most of it is wanting attention. (I believe someone above referenced a 'baited trap', if you will, purposefully seeking negative attention to opportunize[sic] the reaction) Then there's always a possibility of a livable, but nonetheless noticeable psychosis. Possibly body-image confusion, severe overgrooming (maybe not trichotillomania but an offshoot), OC tendancies, or some other mental disorder. Catguy probably falls under that, even though he is an otherwise functional member of society. (should Brian's post be correct, which I don't care to research)
For the record. I have a couple tattoos, one was a pointless "hey I turned 18 lets get tattoos !!!" tattoo, which I sort of regret if only cause it means nothing to me, the others have pretty personal significance. So I 'get' the tattoo thing. Hell I've even had a couple piercings (none of which are still in). And I find tattooed girls insanely attractive to near-constant masturbatory levels. The above however I do not like. (minus the chick with the rack)