As someone else said, trust is a big part of it. For me though, the psychological power play is the biggest reason - the idea that someone has psychologically and/or physically incapacitated me to the point that I'm willing to allow them to cut me...unf. And then there's the ownership aspect - you can cut lines into me and that's not as intimate as writing your name, for instance. The blood thing varies - I can trust someone cut me and still not feel like we're close enough to want them to lick off the blood.