[Castiel:] I gave my word. I have fences to mend in heaven
and as it is, I have a lot to explain.

Sam wasn’t on board with interrogating Metatron, yet he has Dean’s back, and even though Dean almost killed Metatron, when Cas is about to take him away, Sam tells him ‘it won’t happen again’ = I trust Dean. I trust him to do this right. And Dean hears this, and look at the last gif and how he’s relaxing, which will bring him to open up to Sam moments later. He knows Sam trusts him, he knows Sam got his back.