What we think of as ourselves is partially created by the perceptions of others.
"'The wind blows where it chooses, and you hear the sound of it, but you do not know where it comes from or where it goes.'" John 3:8a (NRSV)
Prejudice and stereotypes in the collective unconscious affect us by projecting negative images upon us. These negative images can produce negative behavior in line with the prejudice or stereotype. It is not necessary for these projections to be conscious or manifest.
So what we think about people matters.
If we are thinking negatively about someone this may cause them to behave in a negative fashion.
Of course this is only when our thinking is untrue or false.
If what we are thinking is true then it will have a lesser effect.
But people who are prejudiced think that what they are thinking is true.
So it is better to think positively about people and see the good in them.
This is not to delude ourselves or pretend that bad is good, only to excuse and offer forgiveness.
Then we can value the good in everyone.
(Scripture quotations (marked NRSV) are taken from the New Revised Standard Version Bible, copyright 1989 by the Division of Christian Education of the National Council of the Churches of Christ in the U.S.A., and are used by permission. All rights reserved.)