Castiel appreciated Ben's optimism and just prayed he could live up to it. Letting this go on was simply not an option. Beyond his emotional attachment to the people currently lost, it was his duty to protect them and he would not fail them again.
Still, he hesitated as Mary suggested going inside. It made sense, it was Ben's home after all and where he belonged. But the angel was acutely aware that inside those walls they would be out of his sight and right then that idea scared him more than most things.
"You're welcome," he told Mary quietly, not sure what else he could say, not quite willing to voice his irrational concerns.