In nature you will never see the aggressive hawk and the cooing dove associating.
You ask why?
Simple, two very different temperaments.
Natural law separates the two.
The same law applies to us people.
Two people might be of the same skin, race or religion but inside what they belief is truly different.
You ask why again?
Just like the two birds, the standard of living.
How we were brought up, that is what separates us and causes differences.