First of all, hi! I haven't posted (especially an episode reaction) in forever, but I just had to get in a word about this episode. I have feelings about the situation between Sam and Dean, and they don't seem to be the same feelings that everyone else has.
I hate it when the brothers fight. I am all about how Sam and Dean used to be in seasons 1 - 3. I loved that they loved each other, and that they had their issues, and that they still laughed about things. What people don't seem to understand is that Sam and Dean's lives have changed so much since season 3, there's literally no way their relationship can be the same as it was back then. They're adults now, adults who have lived through decades of literal hell, survived apocalypses, and lost their entire family. They aren't the same people, so their relationship can't be the same.
Dean saves Sam because his fundamental value is family, which is tied up into hunting. For him, one can't exist without they other. He saves Sam because he loves him, sure, but also because he needs him and can't let him go. It's not a fault, it's only human. If you could save your brother from dying, wouldn't you do it? But would you still do it if it was only going to cause your brother pain? That's where Dean seems to have the issue. Sam's point is that every time Dean does something to save him, Sam is the one to pay.
On top of this, Sam is a deeply afflicted character. He is a mess of guilt, self-loathing, and depression. Sam is driven by two things: revenge and penance. Ever since Jessica died his life revolves around those two things. It just gets progressively worse every season. So you have Dean driven by family and hunting, and Sam driven by his own demons. See where this gets messy? Dean either tries to take control of Sam's destiny, which Sam historically hates, or he doesn't support him at all, which kills Sam's self-esteem.
In their current situation, Dean once again did something that took away Sam's choice over his own destiny and it got Kevin killed. To Sam that must seem unforgivable (at this point at least). And now Sam has even more guilt and self loathing over Kevin's death. We saw this when Cas was extracting the left over grace from Sam.
I'm not saying that Sam's right or Dean's right, I'm just saying their given history and the choices made earlier in the season, Sam's reaction makes perfect sense. He must be so, so tired of the life he's living and he had a chance to just let it all go. And before that, he had the chance to shut the gates of hell forever and go out a hero. You can't blame him for being pissed over Dean taking that away from him.
Also, there wouldn't be much of a show without Sam and Dean.