Why do opinions not matter when you don't want them, but when you want an opinion, you get mad at their opinion?

Random opinions are looked down on. Such as saying "Oh I really like this!"

And then another person says, "I'm not into it." Without thinking, but then they realize what they've said. But before they are give a chance to correct themselves.

Your opinion doesn't matter.

Or if you really do mean it.

Girl1 to Girl2: "I really love this dress."

Girl2: "I don't like it a lot."

Opinion doesn't matter.

Girl1 to Girl2: "I really love this dress."

Girl2: "I'm sure you like it, but I'm not into it so much."

Opinion may matter in this situation, but to some it doesn't.

Person1: "Now I need all of your opinions on this."

Person2: "I like it."

Person3: "I love it!"

Person4: "I don't really like it."

Person1: "Your opinion doesn't matter."

If you disagree with someone's opinion or idea, you have to keep your mouth closed. If you make an unwanted opinion; keep your mouth closed.

I feel like I'm contradicting myself in this somewhere, but if you understand what I'm saying; Could you please explain to me why opinions don't/do matter?