Sam does his best not to look crestfallen at Castiel's refusal. He absolutely doesn't want to be an ass and try to manipulate his friend into coming with him.

Instead, he nods, allowing some of his disappointment to show as he straightened from the side of the car and stepped forward, offering Castiel his hand. "Of course I will, Cas. And you know you're welcome in the bunker or just to call any time, right?"