A major reason some extreme Destiel shippers dislike Jared is that many of them did not fall in love with Supernatural itself—they fell in love with a fandom-created version of Dean and Castiel before they ever understood or even watched the actual story. Many entered through fanfiction, fan art, edits, and ship discourse, often starting in seasons 4–5 when Castiel was introduced and Sam was at his lowest point. They focused almost exclusively on Dean/Castiel scenes while ignoring the show's central narrative: the Winchester brothers.
Because of this, Sam was viewed as an obstacle before he was viewed as a character by them. They were not invested in his struggles, growth, sacrifices, or his bond with Dean. What frustrated them was that no matter how much they loved Castiel, Dean consistently chose Sam above everyone else. The story repeatedly made clear that Sam was Dean's most important person.
That resentment toward Sam transferred to Jared. Many fans then searched for reasons to justify feelings they already had, repeating fandom narratives, taking incidents out of context, or accepting negative claims without scrutiny.
Jealousy also plays a huge role. Sam is not only portrayed as exceptionally attractive but also as deeply compassionate, loyal, and self-sacrificing. Jared, meanwhile, was the co-lead of the show, received enormous fan support, and shared the central on-screen and beautiful off - screen partnership with Jensen Ackles for fifteen seasons. To destielers, Jared had everything they wished Misha Collins or Castiel had: narrative importance, audience affection, and Dean/Jensen's primary loyalty. Rather than accepting that Supernatural was fundamentally a story about Sam and Dean, they redirected their disappointment into hostility toward the character and actor who stood at the center of that story.
Among the more extreme corners of the destiel fandom, anti-Jared sentiment often seems to come less from anything Jared actually did and more from resentment that the real story never matched the one they had already built in their heads. In fandom spaces, jealousy often disguises itself as moral criticism, leading people to search for reasons why a person supposedly does not deserve the success, affection, or importance they have.
The more Sam/jared is seen as beautiful, beloved, and fundamentally good, the harder it becomes to explain why Dean/jensen would not choose the alternative narrative they prefer ( destiel/cockles). For some fans, it is easier to attack Sam or Jared than to accept that the show's priorities and reality of relationships on spn set were different from their own.