Something in him (bizarrely) unwinds with every sound Alistair makes. Cullen isn't just a burden, if he can do this for Alistair; he isn't worthless; he does give Alistair something he wants.

"Good," he breathes, hard, against Alistair's throat. "Mine. You're good." It helps to stop and catch his breath, even if he's still rocking slowly against Alistair in a steady rhythm. "So good to me."