It’s that Dean and Sam are so fucked up.
I mean, Daddy Winchester says ‘protect Sammy’ and Dean does it for his whole life until Sam says ‘I can’t stay here any longer’ and suddenly Dean’s missing half his directive. He used to be ‘protect Sam. Follow Dad’s orders.’ Then he’s just ‘Follow Dad’s orders’, only, Daddy Dearest isn’t around much. So he’s not even really that.
Then, ooh, look. He gets Sammy back, only it’s ‘Sam’ now and Sam neither wants, nor really needs to be protected.
And then Dad comes back. But Dad changes his orders. It’s no longer ‘protect Sam(my)’. No, now it’s ‘watch Sam’. Save him if you can, but watch Sam. Watch him and, if you have to, kill Sam.
Dean’s gone from ‘protect’ to ‘be ready to kill’.
And Sam? Sam just lost the one person who, no matter what he did, he knew had his back.
All of this? They’re still the only people they can trust.
That’s what gets me.