Why is hard to change somebody's mind or yours?

Normally when someone has decided that their mind has been made up. It is very difficult to change it. Our minds have been trained to keep an idea and not let go. Our minds tend to refuse most facts. So instead of having, lets say and opinion, that is based off of facts it is made up of emotions and how that 'opinion' makes you feel. The chemicals in your brain makes it hard to let go of an idea or opinion.

Because people believe what they heard first or growing up, people have a set idea of something and people are stubborn.