Castiel really is beautiful like this. Crowley looks forward to pulling him apart bit-by-bit with his mouth. He's seen a lot of people fall apart, but he already knows that Castiel will be the most impressive. He's open and honest in his reactions to a fault and Crowley's already seen him desperately rub himself on someone's leg.

"Relax, angel. I've got you." It's as much a pet name as it is a teasing reminder of just how different they are and then he takes Castiel into his mouth with practiced ease. He's generally more for recieving than giving, but he's no stranger to it and Castiel is certainly worth the effort.