system
A chat on subreddit /r/NoStupidQuestions
human
So I am a liberal so this is from that biased viewpoint. But for those policies what the general arguments would be:

* Wanting to defend our jobs. A lot of people come in and immigrants cost a country money in the short term (they do gain it in the long term but that's not looked at as much) and take low paying jobs that could otherwise go to someone else. They're also often willing to work for very little driving the wages down lower. There's also a fear over illegal immigrants receiving government services or general money despite not paying taxes. 
* Most republicans and most of the right wing wouldn't call themselves racist. They view it as racism and racist policies were a thing of the past and we should all move on and not talk about race. The problem is a lot of those underlying issues haven't been fixed yet so the idea of moving on before fixing them just perpetuates the problems. 
* With abortion they see it as life begins at conception. And so aborting a child is killing it. I really understand that position even if I don't agree. 
* They're for lower taxes for everyone, but often tax cuts benefit the rich most. There's also a strong American mindset of the American dream. I may not be rich enough to pay an estate tax now, but when I make it big then I will be or my kids will be. That's not carried out very often in facts but it's a belief. If you see yourself as soon to be rich, then having taxes favorable to the rich makes sense.
* Socialism as a concept isn't one well understood. It's often conflated with communism so the two get used interchangeably. So being pro socialism is seen by them as being pro soviet communism. They also often know very little about the situation in Europe. They view socialism not like how it's working in Europe but like how it isn't working in Venezuela. 

Edit* to be clearer I understand Europe is not socialist. But it does have some socialist policies like healthcare that work very well there. And when similar policies are suggested in the us they're being compared to Venezuela not Europe. 

* Gun control, they like owning and shooting guns and don't want to give them up. It's also written into our bill of rights. 

I think too often both sides view the other side as a red herring and look at the worst form of the argument the other is making rather than the best forms. And it's very hard to see why someone else would believe in this red herring of the worst form of the argument. Obviously you wouldn't believe that it's stupid. But you most people's viewpoints can be understood even if you don't agree with them, or even if they're not always logical beliefs.
gpt
This is why actual discussion is important. People rarely are able to fully flesh out the problems with their ideas unless they discuss them.

People on the left have good ideas and bad ideas.

People on the right have good ideas and bad ideas.

They can not figure out which are which without interacting with people who have different perspectives.

This is why partisanship only benefits the political establishment, lobbyists, and corrupt institutions. When discourse is shut down and everyone is divided into echo chambers, they can exploit the flaws in every policy and prevent them from ever being addressed or corrected effectively.

It also allows them to say "Hey none of this is working, but we can do better if we centralize more power and authority into fewer hands!"

Which results in authoritarianism.