What reasons might people have for not give their opinions in questionnaires?

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I think the most common reason will be not willing to share privacy in a questionnaire. Nowadays questionnaires are long and cover almost everything in our lives, so they refer to many privacies. Secondly, questionnaires tend to be longer and longer in recent years so I think people are unwilling to spend much time on fulfill them. What do you think? What reasons might people have for not give their opinions in questionnaires? :)