A lot of time our opinions on things are formed before we ever experience them. It applies to new food, movies, and even people. The scraps of information we've gathered before experiencing something tend to shape our first impressions. Your experience with a new video game can start from a negative viewpoint before you've played a minute of it. Then it can either surpass your lowered expectations or be exactly what you expected.
We do the same with people. You can hear a fact about someone or a story and an image instantly forms in your head. Even if you're aware you don't have enough info to form these beliefs, it's hard not to anyway. When you hear someone's vegan or an ex-con the thoughts you've associated with those things in the past resurface. If you're used to vegans who lecture about veganism, you bristle at the thought of another vegan potentially doing the same.
Our brain is the culprit, like a good programmer, it's lazy. It's always looking for patterns and attempting to apply solutions that worked in the past to new problems. Those shortcuts have helped our species get to our current position atop the food chain, but it can affect our existence in modern society.
Survival is no longer our main priority so what worked in the past doesn't work as well now. When we form preconceived opinions we limit ourselves and our own experiences. We're sceptical of new things and unwilling to give it a fair chance. What you expect can influence what you get. At their worst, they can lead us to accept stereotypes.
It's not easy to ignore these natural tendencies but we've progressed as a species by doing just that. We gave up the hunter-gatherer lifestyle to settle and farm. We gave up some of our agency to others to form functional societies. Those changes were several orders of magnitude more significant.
On a smaller, personal scale, I'm trying to have fewer preconceptions about everything. It lessens my tendency towards cynicism and I'm more open to new experiences. It might not be as important a change but I think it can have a noticeably positive impact on your life.