Big Willie It's a cognitive dissonance thing, innit.
Edit: I don't necessarily endorse everything that follows but I feel it's a decent effort at explaining a well known theory about this stuff.
To put it in a blurrily Freudian way, when people notice there's some suspended conflict or threat in their world, they adaptively ruminate on the factors of their stakes and safety in light of this threat, feeling pleasure or anxiety depending on whether these factors seem to be strengthened.
If your brother's accused of tax fraud and you support him, you're happy with every circumstance that determines his innocence.
Except this determination isn't neutral, it's demonstrative and integrates any sign the power of any threat of consequences for your brother or your family is receding, so it includes indirect support like your mates at the pub letting your brother buy a round, and also your interest in the outcome gets tangled up with dependent stakes like the promise your brother made to help you renovate your house.
The famous psychoanalytic insight is that in our human social setting, the very basic adaptive system we use to understand the factors affecting us is not only about about what we believe is true. It's often about what we believe other people believe is true. This isn't some kind of mistake or illusion: the kind of truth we usually have to care the most about is a real but social truth. What we understand about what other people believe is often more concretely important to us than what we ourselves seem to believe.
In a lot of cases, what we "privately" believe is almost irrelevant to us and fades from view, or we think we believe things but defer belief to "what everyone thinks" or "expert opinion".
In the hypothetical, maybe you have hard proof your brother committed tax fraud—you saw the emails and the cooked books yourself—but you still feel powerfully that he's innocent, because that ends up being what everyone else thinks, that means he doesn't go to jail, and so it means your brother's real and effective innocence is more a matter of other people's beliefs than your incontrovertible facts.
This is almost the opposite of the popular catchphrase of gritty, hard-nosed, hairy-bollocked pragmatic realists: turns out the feelings don't care about your facts.
The destination can be an uncanny situation where it's a year later and your brother's been acquitted of tax fraud, and you're both painting a wall and you're really enjoying rolling it all out, because due to all the thinking and ruminating you did on the whole lousy situation with his trial, seeing the green paint spread on the wall feels like you're proving your brother is innocent.
This feeling of relief goes along with your adaptive system no longer perceiving the threat that set this all in motion.
Don't think it's unfair to claim the "Partey discourse" has shown signs of all this on all sides, varying based on what different people feel their stakes in the situation to be. Some of the varying dimensions of the stakes include the way Arsenal's successes or failures impact them, impact Arsenal's perceived classiness, impact the way people perceive them as Arsenal fans, create the perception of having won an argument with their mates, and so on.
And when circumstances are lining up with the stakes you've established, it's like the whole world is announcing your relieved "I told you so".
