That sensitivity is the main reason for the chastity device. Castiel would come too quickly, be too sensitive afterward to enjoy things. And while Sam loves that sensitivity most of the time, it's good for Castiel to be able to enjoy more, too.

He hums, finally reaching down to undo his belt, making certain to make enough sound for Castiel to hear him. "Wish I could, too, Cas," he says, licking his lips. "Bite and suck on them until I decided to put the clamps on them."