Why are Duo Queue's so often toxic?

Obviously not all duo queue's are toxic, but so many I see are. They tend to never let anyone constructively criticize either of them, but if you mess up, there will now be two toxic people berating you for your mistakes. They also always think that whenever a game is doing poorly, its not in any way them, or their partners fault. And when you try to surrender, they tend to both vote the same, and if it is no, it is likely you wont ever convince either of them to vote yes. This is all my opinion though, so I could be wrong.