You know, we all talk about how Dean wouldn't just leave Cas in The Empty after 15x18 and like...I agree but also, I feel like Sam would actually be the driving force behind getting him back.

1) because he's seen how Dean gets when Cas dies. He knows it gets worse each time and he is not about to see where this time leads him.

But more importantly 2) Sam has no idea wtf happened. There is no way Dean told him the details. It would have simply consisted of "Cas sacrificed himself to the Empty to get Billie off our asses" and ANY attempt at Sam trying to get more out of him would just result in a "He's GONE, Sam" and Dean storming off. And Sam haaaaates not having all the information. In his mind, Cas has already come back from The Empty once so why the fuck can't it happen again?