People's views are heavily influenced by political tribalism.
The simplest form is probably something like “my mom and dad and college buddies all vote [Republican/Democrat] in every election, so I'm going to do the same without even paying attention to the actual policies,” although it can take far more extreme and more malignant forms.
A queer person might notice that Western millennial socialists tend to be the 'tribe' that's most outspoken in terms of LGBT rights, so they decide that socialists are probably right about everything else too. Before long, they're calling for the government to abolish private property and nationalise all industries, despite the fact that they've never given much thought to the intricacies of political economy.
An anti-war advocate might notice that libertarians are the 'tribe' that's most outspoken about opposing U.S. military intervention in foreign countries, so they decide that free-market capitalist libertarians are probably right about everything else too. Within a short time, they're supporting massive tax cuts and across-the-board reductions in government spending, even though they haven't actually considered the consequences of those policies.
I'd strongly encourage people – especially those who are relatively new to political engagement – to be extremely sceptical of package deals and ideological buy-ins.
However, there's an even worse motivator than tribalism: reverse tribalism—based in opposition to an enemy tribe rather than support for a friendly one, rooted in negative associations rather than positive ones. I consider this worse because, quite frankly, hate and anger tend to cause irrational and destructive behaviour more than love and loyalty do.
Sadly, in the modern era, this seems to be even more common than normal tribalism, and I see examples of it everywhere. For example, “A Bernie Bro insulted me, therefore I should completely dismiss anything the progressive leftists say and support centrist Dems over progressives in every primary election.” Or “I talked to this one libertarian who was a pedantic jerk ranting about conspiracy theories, that's why I've concluded that libertarianism is a garbage ideology and all libertarians must be deranged crackpots.” Or “My uncle was a staunch [Republican/Democrat] and he was always really mean to me and my siblings, so I'll never vote for that party, it must be filled with scumbags like him.” Or “This Social Justice Warrior on Twitter criticised me for being privileged, it's clear that the whole Social Justice movement is just an excuse for status-obsessed millennials to score wokeness points and tear down their rivals with targeted callouts.”
This can also take a slightly different form, where people look at someone's stances on certain issues and then make unwarranted assumptions about all of their other stances. A LGBT rights activist might notice that Republicans tend to be anti-gay, and Republicans also tend to favour lower taxes, so they'll conclude that anyone in favour of lower taxes must be a far-right ultra-conservative quasi-fascist Republican bigot who hates gay people.
A Second Amendment advocate might notice that Democrats tend to support gun control, and Democrats also tend to support increased welfare spending, so they'll conclude that anyone in favour of increased welfare spending must be a far-left quasi-socialist Democrat tyrant who wants to take away their guns. (To those who object that “statistically, someone with Position X is more likely to hold Position Y, so it's not an unreasonable assumption,” I'd reply that those correlations are smaller than you probably think, and certainly not large enough to warrant assuming bad faith of people you don't know.)