The false consensus effect makes you think your opinions are more common than they are

Lee Ross at Stanford showed in 1977 that people reliably overestimate how many others share their beliefs, preferences, and behaviours. Students who chose to wear a sandwich board on campus estimated 62% of others would too; those who declined estimated only 33% would agree. The bias arises because we disproportionately associate with people similar to ourselves, and because our own view is the only one directly accessible to us. It explains why politicians misjudge elections, why cult leaders expect mass conversion, and why people are shocked when their 'obvious' opinion is contested.

Source: Ross, L., Greene, D. & House, P., 'The False Consensus Effect', Journal of Experimental Social Psychology (1977)