Because I research my opinions and don't rely on someone on twitter who whispers sweet nothings in my ear trying to get me to subscribe to their patreon.
Obviously anybody's opinion is going to be the one they think is most correct, otherwise they would pick the opinion that they think is better. It's like saying something is always in the last place you look for it.
Why is mine correct? I don't know that it is. If I find a better opinion I will discard mine.
Looking deeply into why someone believes what they believe usually winds up not being very deep in the first place. I take in information through experience and external sources and I make sense of things by thinking about them.
It's not and it never will be. I accept that my opinions are just that. They're my opinions. Whether they're right or wrong depends on how everything plays out in real life. Even if they make sense to me, they most likely won't make sense to most other people. Even if people did happen to agree with me, that still doesn't mean my opinions are inherently correct, good, or whatever.
My prejudices seem to have a funny way of being justified sooner or later. This sometimes causes me distress when I suddenly develop a prejudice against something and can't immediately explain why I feel that way.