People flock to other people that make them feel the way they want to feel, and tell them what they want to do is okay.
People find these groups organically too, or form them themselves, through friend associations, not just by having them pushed.
A lot of people like to feel angry against someone (the other), as it gives them a sense of control and power. Joining others who are similarly aligned, also gives them the sense of belonging.
People who are in these states aren’t going to filter out their friends, or filter out the people telling them things they want to hear.
They’re going to filter out things that make them feel uncomfortable, or that they don’t agree with - like anyone who would stop them from going further down the rabbit hole!