The New Statesman has an explanation of why the left are so stupid:
Correcting and decrying the perceived wrongness of others is the chief political preoccupation. David dismisses Ed, Andy elbows Harriet, Jeremy and Nigel condemn everyone else and on it goes. People who say they dislike this denounce others as pursuing yah boo, negative politics and these people are, in turn, are impeached for failing to provide proper opposition.
If the goal is to persuade people to change their minds, then attack and rebuttal has a mixed record. On one extreme, SNP attacks on Scottish Labour clearly moved voters dramatically at the last election. But on the other hand, think of all the criticism that a George Osborne or a Harriet Harman has faced in their careers: it hasn’t seemed to have really held them back from their political projects.
One explanation may be found in the idea of “backfire” or “boomerang” effects that students of persuasion often talk about. Being told that Protestants are conservative can make Catholics more liberal. Saying that vaccines have no side effects appear, at least sometimes, to reduce the chances of someone getting a vaccination. No smoking signs can increase the desire to smoke. Informing American conservatives about the dire consequences of climate change on France has been found to reduce support for addressing carbon emissions.
Once we hold an opinion, it seems that we will try and defend it. Suppose you tell me all the reasons why you hate the West Wing. I’ll take the time to mentally dismiss each one of your criticisms: why shouldn’t every major political issue be resolved with a 90 second speech and some rousing music? I’ll begin to delve into my memory for my favourite episodes. By the end of the conversation, I’ll have applied myself to building the case that the West Wing is brilliant and my own counter-arguments may stick with me for much longer than your criticism.