The criticism is mean-spirited. If you use insulting or degrading language or put down the person in any way, they will focus on that, and not on the rest of the criticism.
If you focus on the person instead of their actions, you will make them angry or defensive or hurt.
They assume you’re attacking them. Some people can’t take criticism in a detached, non-personal way.
They assume they’re right. Many people don’t like to hear that they’re wrong, whether it’s true or not.