Stereotypes are dangerous because they assume that all people of a certain group are exactly the same, removing their true identity and and assuming their are resistant to change. Stereotypes are statements about a group of people that tend to over-generalize, giving others an inaccurate and often irrational representation of the group.
When most people think of stereotypes, they think of negative assumptions, but positive stereotypes can also be dangerous. Examples of common positive stereotypes include all Asians being good at math and all African-Americans being good at sports. Positive stereotypes influence how people see the nature of the differences in a specific group of people and this can, in turn, trigger negative stereotypes.