I contemplated what's gone before and started to see a pattern.
I became aware of fandom around about the time of Asylum (1.10). One of the first things I read was a post defending Sam against the hate he received (yep! it started way back then) because of the way he turned on Dean ("you're pathetic", "good little soldier" etc.). At the time it puzzled me because I was thinking about how Sam must have felt to be made to say those things to his brother, but the reaction was focused on how Dean was hurt by what Sam had said. Sam attacked Dean and his place in the family and therefore crossed a line. Sam's experience of being attacked by a supernatural being was quickly forgotten as the focus became on what he said to Dean.
Out of all the things that can hurt Dean, it's Sam that seems to hurt him the most. Either from his actions (any time he leaves Dean for any reason - including death) or by his words. Sure, Dean is hurt by words from his father, the consequence of some of his actions, his constant plague of doubt and self worth, but the deepest hurt seems to have Sam at the heart of it (Death being the most notable, but also any time he's left Dean or said something that hits close to home).
That's not to say that on the flip side most of Dean's joy is from Sam. Dean is content with simple pleasures in life - his car, pie, a good hunt, friends and Sam. Sam can make him smile, be bursting with pride and fulfil much of his feelings of being needed. Dean wants to be a good big brother to Sam. Much of Dean's self worth is wrapped up in Sam's well being. Dean would do, and has done, anything to make sure Sam is around.
So it's no wonder that when we see Dean hurt by Sam we tend to feel sympathy for Dean. He's the one trying to be the best big brother. He's the one who sacrificed his life for his little brother. He's the one who wants to protect him and look after him.
And all Dean really wants in return is a little brother who looks up to him (the very thing the siren from Sex and Violence knew was a deep desire of Dean's) - it doesn't seem like a lot to ask.
But we can't look past the fact that Dean is also the one who hurts Sam the most.
For some reason Dean hurting Sam seems to be ok because Dean does it with good intentions. Dean makes Monumental Statements of Love (MSoL) by doing things like selling his soul so his brother can live, calling on Death to return Sam's soul and, most recently, bringing Sam back from the brink of death with a healing angel. Dean does everything we love and want to see from this relationship - major sacrifices for love.
Sam hasn't been given one of those Monumental Statements of Love (yet). Sure, he's had lots (and lots!) of little ones but never that BIG one that this brotherhood seems to be measured on. And this, I think, is at the heart of why Sam is often vilified. He hasn't proved that he loves Dean as much as Dean loves him. He hasn't made that monumental sacrifice to save Dean.
Dean ends up hurting Sam through actions that are the result of a love (and need) so strong that death won't stand in the way. Sam ends up hurting Dean because he reacts or has been affected by Dean's action. Dean does it for love and brotherhood and family and therefore it's deemed to be ok. Sam does it as a result of being physically or emotionally hurt and therefore it's not ok. Dean does it for all the right reasons - family. Sam does it for the wrong reasons - himself. It's therefore seen as selfish and whiny and ungrateful. Standing up for yourself or voicing an opinion is ok, as long as it's not against the person who has made a huge sacrifice for you.
Also, we rarely get to see into Sam's inner thoughts. It's often been observed that the show is seen through Dean's eyes and I think this is true. We see why Dean is hurt, we mostly have to guess why Sam is. It's most definitely a story of two brothers, but it's often about something that's happening to Sam (dead Sam, soulless!Sam, blood addicted Sam, trials!Sam etc) that Dean has to react to (grief - sell his soul, desperation - make a deal with Death, despair - kill Ruby and rescue Sam, angst and grief - heal Sam by possession).
What happens to Sam after Dean's MSoL usually happens over a long period of time. By the time Sam has suffered and then reacted we are no longer focussed on what Sam has been through, but rather that he's said something that shows that he doesn't appreciate what Dean has done.
If we look at what's happened to Sam as a result of Dean returning him from the dead in S2 we can see what Sam has had to endure. He lived a year knowing is brother was going to die, followed by 4 months of grief after the loss of Dean, he then lived another year addicted to demon blood (and subsequent withdrawals when he was recovering), he lived another year trying to redeem himself after releasing Lucifer (and losing Dean's trust), he lived 6 months without soul, then had to make peace with what he did when soulless. He then suffered with memories of his time in the cage and hallucinations of Lucifer.
But none of this really matters because it doesn't measure up to the MSoL. We acknowledge what Dean did to save Sam, not what Sam suffered as a result of it. We acknowledge it because we love it. It demonstrates the lengths Dean will go to for Sam. We reveal in the awesome brotherhood bond. We see this as the heart of the show. We love that Dean needs to have Sam around - it's heartwarming and much of what the show has been based on for a long time. The problem is because this very thing is the "heart of the show" anything Sam does outside of that pales to insignificance. His suffering doesn't matter because he's alive. His angst doesn't matter because by being alive the brotherhood continues and because it's the reason we watch there's nothing Sam can say or do that can be more important than that. You were used? Too bad - you're alive, be thankful. You want to make your own choices? Stop whining, you're alive, the brotherhood lives so shut up.
Season 9 is no exception. Sam has suffered this season and the only thing that seems to be remembered is that Dean saved Sam from dying. Dean mentions family and we are reminded that this is what this show is all about. Our hearts melt and nothing Sam does matches up to that. In fact, when Sam says something against what's at the heart of the show he loses all sympathy and everything he has been through is forgotten.
Forget that he was prepared to die to close the gates of hell, forget that he made peace with death so others wouldn't be hurt if he lived, forget that his brother didn't trust him with the truth for fear he'd make the "wrong" decision, forget that his mind and body were violated (something he has suffered before), forget that his body was used to kill Kevin, forget that he'd be despairing from learning this knowledge.
Sam questioned their brotherhood. He questioned Dean. He questioned the heart of the show. He questioned everything we hold dear about the show. He's therefore easy to blame when looking for someone to take our frustrations out on. The brotherhood is broken. Sam says it's broken. Therefore it's his fault. What's that about not shooting the messenger?
Even those who agree that Sam has a right to be mad at Dean's deception, can't accept that Sam questioned their brotherhood. That's crossing the line. Dean can cross a line by allowing a non con possession, but Sam can't cross a line by being mad about it OR by acknowledging that there's something broken in their relationship. It's too much for Dean to bear and it would seem that fandom has trouble coping with it also.
And I'm not arguing about this because I would want anything less than a show that's about a strong brotherhood bond. I watch the show for this. I love the show for this. I watch for the bromance and desperate love between these messed up brothers. But the show is coming into its 10th year (amazing!! \o/). How much longer can we watch Sam suffer for Dean's MSoL? I know many will say FOREVER! BRING IT!! And that would have been me once, but now that the show is highlighting this cycle of hurt through sacrifice I want to see how it will ultimately be addressed. I want to see it acknowledged that both brothers have played a part in why their brotherhood is broken and I want to see them both play a part in fixing it (and as much as I know this won't happen, I'd love fandom to be able to see both sides of this current conflict).
Of course, I'm not saying everyone feels this. I know many can see Sam's journey and sympathise with it - even when he lashes out at Dean. I know Show makes it a lot harder to sympathise with Sam, especially when giving him such damaging dialogue (Sharp Teeth, The Purge), or showing how much he hurt Dean when growing up (DSotM), to telling Dean he doesn't want things the way they used to be (Shadow) and having him leaving Dean when he's upset (Hunted, The Girl Next Door), or acting cold and uncaring (soulless!Sam).
But Sam's motivations aren't simply to hurt Dean for the sake of it. He doesn't do it to be spiteful or deliberately "mean", or because he has attitude, or is whiney or selfish. He always has reasons (they're just a bit harder to see sometimes).
For me, Sam doesn't need to prove his love for Dean. It's constantly on the screen. Much of what he does and says is all part of the drama and forms part of the overall picture that is the Winchester relationship. Which, to me, is (and always will be) the heart of this show (even when they are in conflict).