That hurts Castiel, not because Dean doubts him, but because Dean is hurting so much.

"It's really me, Dean." He leans forward, resting his forehead against Dean's for a moment. "I'm sorry I wasn't faster."

He gives them both that moment of just touching--connecting--and then he gets back to freeing Dean. It must hurt, but he knows that not freeing Dean will hurt him more. It's best to force through it.