Here is one of my favorite quotes: 

If I dislike someone, 

the way they hold their spoon  

will offend me.  

If I like someone,  

they could drop a plate of food  

in my lap and I wouldn't care. 

Our preconceived notions of who we think people are keep us seeing them as we envision them - and, perhaps, from seeing them for who they really are. 

Perhaps people in our lives don't change because we insist that they are a certain way?

Spiritual-religious advice: Be aware when you are creating stereotypes - even little ones.