When he felt the effect of the magic, James couldn't help but being attract by it. It wasn't the sensation, although it was alluring as well. No, it was his natural curiosity toward new magic, toward things he hadn't experienced.
He let the sensation wash over him, learning it, memorising how it felt, studying it how the warmth could easily take over. He studied every nuisance of Severus' magic until he could almost picture Severus' will.