His arms are loaded with scars from where he used to cut himself, not out of desperation, but as a party trick. Skin is all there is between you and the rest the world.
Blood is so eager to get out.
That's Anna's idea of a come-on, and her way of inviting Sam to tumble into chaos. From the moment she first jerks him off in Battery Park, Sam is obsessed with her, driven headlong into self-destruction. He can see that he's destroying his relationship with his ex, that his friends don't like him as much, that he's paler and his health is failing, but he just watches as his world falls apart. Habit is also a vampire movie, and not just because Anna likes to bite Sam and suck his blood during sex. Sam's grasp on self-control is as tenuous as the thin layer of skin which separates blood from world, and when Anna bites through it, she's only unleashing Sam's own death wish.