Sam and Dean Winchester spend so much of Supernatural's 15 seasons lying to each other and arguing that it is a point of humor for the ionate fandom. While Dean does some pretty awful things and seriously betrays the trust of Sam, the younger of the Winchester brothers also does some questionable things that result in the betrayal of Dean.
Whether it be choosing someone or something else over Dean and the family, making a mistake that costs someone their life, or just making generally bad decisions -- all of which both brothers are guilty of at times -- Sam does a lot of stuff that hurts Dean.
When He Leaves For Stanford
There are some betrayals in the series that fans feel are justified, and perhaps aren't even actually betrayals, but just something that the other brother is hurt by. Sam leaving for Stanford is one such instance.
While the way Sam went about doing so was questionable, the actual idea of Sammy going on to college to live a happy life was not a bad one, at the start of the show at least. It did hurt Dean, though. The older Winchester felt abandoned and hurt. All Dean ever wanted, from the outset of season 1 to the finale, was his family, together, fighting with and for one another, and Sam left Dean to deal with John's nonsense all on his own.
When He ed The British Men Of Letters
There are some betrayals in the series that are easily forgotten by fans or don't get talked about enough but are still shady. This is pretty much the case when Sam s the BMOL.
While it is understandable why Sam ed, it was still wrong, and it was pretty shocking that he didn't tell Dean and instead lied to him for weeks on end, all while trying to recruit him. Dean understood the track record of BMOL at that point, and so when Sam finally unveiled the truth, Dean was hurt about the lies, but not necessarily about Sam ing the BMOL itself.
When He Didn't Tell Dean He Was Back From Hell
Soulless Sam was a storyline that impacted Supernatural deep into season 7 and was also one of the most unlikable iterations of Sam. He hurt Dean right off the bat with his casual announcement of being back from Hell for a whole year.
Of course, his reasoning was sound -- he wanted Dean to enjoy his new life with Lisa and Ben. But this reasoning alone proved it was not quite Sam talking. Dean was rightfully hurt, having spent a year mourning his brother. Sam cannot be properly blamed for this obviously, as it's just one of the evil things he did against his will, but he still did it, and Dean still had to endure it.
When Soulless Sam Let Dean Get Bit
It is difficult to say that Sam betrayed Dean at all while soulless. As Dean pointed out many times, this wasn't Sam, just a shell of his brother. But Dean still had to go through a lot dealing with his soulless brother, not least of all when Sam let Dean turn into a vampire.
The real Sam would never have allowed Dean to get turned into a vampire just to find a nest, but soulless Sam was a different breed of Winchester, and his ruthlessness was only really matched and sured by Mark of Cain/demon Dean. Dean being a vampire pretty much cost him Ben and Lisa, which would prove to be his one shot at domestic happiness in the show.
His Happy Memories In Heaven Didn't Include Dean
In "Dark Side Of The Moon," one of many excellent season 5 episodes, Sam and Dean end up in Heaven after being shot by a pair of hunters. While there, they have to navigate their own memories to reach Joshua and perhaps God.
While Dean's memories involved a Fourth of July with Sam and a typical lunch with Mary, Sam's had less to do with his family. All of Sam's memories were either on his own, having escaped Dean and John, such as the night he left for Stanford, or when he lived on his own for a little or were with other people, such as the other family he spent Thanksgiving with. While Sam was not in control of these memories in Heaven, to say they hurt Dean is an understatement.
The Book Of The Damned
Sometimes Sam and Dean's urge to save each other from a doomed state, by any means necessary, costs lives. Kevin died when Dean allowed Gadreel to possess Sam, and Charlie met her brutal end far too soon when Sam would not give up the Book of the Damned.
Desperate to find a way to get the Mark of Cain off Dean, Sam continued to use the Book of the Damned, despite telling Dean it burned and despite Dean's pleas never to use it. Dean was out of order telling Sam it should have been him to die instead of Charlie (but that can arguably get chalked up to the Mark of Cain), but Sam's insistence on using the book was sheer desperation that resulted in the death of someone Dean considered a sister.
When He Told Dean They Aren't Brothers
When Dean allowed Gadreel to possess Sam and the younger Winchester eventually found out the truth, it resulted in perhaps the biggest falling out between the brothers ever. It eventually saw Sam tell Dean, "You want to work? Let's work. If you want to be brothers ... those are my ."
Sam seemed to truly mean it. He told Dean he could not trust him - which is understandable - and says that all their problems have come about because they are family. This hurt Dean to his core. The entire ordeal only seemed to truly resolve itself after Dean died at the hands of Metatron. Until that moment, Sam was practically done with Dean on a personal level.
The Demon Blood Lies
Sam often gets a bad rap for the stuff he does in Supernatural, and it is often unjustified for him to get so much hate. But in season 4, any criticisms of Sam were utterly justified when it came out that he was drinking demon blood.
After lying to Dean and keeping him in the dark for so long, when Dean finally discovered what Sam was doing in "The Rapture," he struggled to see his brother. It was almost naive for Sam to allow himself to drink demon blood and become addicted to it, especially considering some of the awful stuff he did while addicted. The lies and secrecy from him hurt Dean nearly as much as the drinking of the blood itself.
When He Chose Ruby
Long before Dean discovered the truth about the source of Sam's powers, he discovered Sam was working with Ruby to develop the powers. That in itself was a betrayal to Dean, but something he got over as season 4 progressed.
At the end of the season, it gets so much worse, though. After tracking Sam down and fighting him, Dean tells Sam if he leaves, he should never come back, and he leaves anyway. Sam chose Ruby time and time again in season 4, and that final instance was the worst for Dean. While Bobby's great speech about family and his outburst at Dean for giving up on Sam was justified, it is understandable why Dean felt so betrayed by his brother.
When He Didn't Look For Dean In Purgatory
Dean was never the same when he returned from Purgatory. From his return onwards, he was more adamant than ever about his fate being death on a hunt, something fans did not want for the character, and he was suffering, something made worse by Sam's revelations upon their reunion.
Dean undoubtedly expected a Sam who had been searching far and wide for him, while watching over Kevin and Mrs. Tran for a year. What Dean got, though, was a Sam who hit a dog, fell in love, and - to Dean - forgot about him and Kevin. So many fans hate the Amelia storyline, and with good reason. It seemed totally against Sam's character to not look for Dean, and Dean held a lot of hurt because of it.