As hard as seasons 9 and 10 are for me to watch (simply because I hate seeing Dean go so dark, not because I don't like the seasons because they are phenomenally well written and acted), one thing I do absolutely love is the Cas/Sam chaos that we get. Those two are just pure calamity together and I love it lol. With Sam's being a magnet for trouble and bad luck (not to mention his staggering levels of self-sacrificial tendencies and low-self-esteem combined with a desire to always do the right thing), and Cas' adorable lack of understanding things, every time they get together over this arc, it is the most chaotic, shenanigans and I live for it. Much needed levity for the darkness and heaviness of everything else involving the Mark of Cain. But yeah, I completely agree, they should never be left alone together lol. Competent as they are on their own, left alone they have like 3 braincells between the two of them lol