I need more input on this. In your eyes what is the base of an opinion? the underlining characteristics?

I have been talking with friends and thinking quite hard on this topic since I seem to be a bit of a nerd and am quite fascinated by "opinion". How it forms the base of so many qualities in a human, a society, and even a country. So please i would like some more input on what you think is the underlining characteristics of an opinion (which is an opinion within itself.)