You don’t understand though.
Dean was holding Sam’s head as he died. He died right there in his two useless hands. And he was still holding Sam in his hands when Jack swooped in and brought Sam back. Jack brought Sam back to life like it was no big deal. All Dean cared about in that moment and forever is that Sam breathed and moved and was...alive again. And he can’t believe it, the luck they have, to keep dying and getting another chance. He was holding Sam while he lived, while he struggled to keep living, while he died, while he was dead and gone, and when he was resurrected. His hands aren’t useless, they are witnesses to the miracle that is Sam.