Why is it so easy for the insensitive habits to be repeated? Is it a self-esteem or self-protection thing?

Meet the people who can't explain how and why they got more prejudiced against other kinds of human beings ever since they got older and / or got married and had kids. 

They used to be less closed-minded, but things are different now.

Is it a self-protection mechanism?

Did they ever subconsciously feel their kids or perhaps their grandkids would need to see an example they don't or would not want to repeat?