R is Dead. Capital D. He's a cold body. Julie is Living. Capital L. She's a warm body. R wants to be living, to have a heart beat, a pulse. He doesn't get this feeling from his inherently destructive behavior of killing people and eating their brains, as fun and tasty as that is.
He gets that feeling from lurv. Only by loving Julie does he get all filled with ooey-gooey warmth and become a warm body himself. "I feel it. A movement deep inside me. A pulse" (2.8.239). He's a real boy!