>Someone expressing his opinion is not equal to starting an argument, in any context.

Expressing an opinion as though it is fact, in such a way as to deprecate something or someone, is an attempt to start an argument, whether intentional or not. Anybody associated with the thing being deprecated risks loss of social status, and it is natural human instinct to want to avoid that. If I say "I'm right and you're wrong", that is an attempt to start an argument. In most contexts it would be entirely reasonable for you to reply disputing it, and nobody would claim your were "starting an argument" by doing so. The one who makes the deprecating claim is the one who starts the argument, not the one who replies.

I do not object to argument per se. Arguments can be a valuable part of discussion. But if I say "I'm right and you're wrong" in a context where any attempt you make to defend yourself would be downvoted no matter whether it was correct or not, then that is the wrong context to make the claim. There is no value to an argument under such conditions.