Because Dean has to deal with Sam 24/7 and I imagine his patience runs out, for Sam and for all the other stuff he's dealing with on top of Sam (which are all Sam-adjacent anyway). There's barely any room to breathe with that kind of stress. It's tiring. But Cas comes in with fresh eyes, if not an entirely full heart. Robosam, to Dean, is everything that is wrong with the world. Dean expects a lot from Sam, and from Dean&Sam. Cas doesn't necessarily have these expectations, and he definitely doesn't have that baggage.

Also DO YOU THINK SAM'S SOUL PRAYS TO CAS.
NOW I DO.