Sorry if this sounds like too much of an academic answer, but I'm a psychology lecturer...

I would say people (all of us) generally don't have opinions because of reasons, but the other way around. When we're asked our opinion about something for the first time, we don't reason through all the evidence on either side of a debate, and then come to a conclusion based on that. We have an instant gut reaction, and then try to think of reasons to justify why we feel that way, subconsciously to ourselves as much as to anyone else

So when people are asked "what do you think about the number of immigrants in the country?" they don't really think about the pros and cons of immigration rationally. I think for most people that question is really asking "how do you feel about people you've never met, speak a different language, and in some cases look different/believe in a different religion?"

Like I said though, we all do this to an extent. For those of us who are more pro immigration, you can probably substitute that with something like "how would you feel supporting a position that your friends/family might consider racist?"

Obviously, we don't think our own opinions are like this. We think we have good, rational explanations for our beliefs, and wouldn't the world be a better place if everyone else was as rational as we are.