Why do people enjoy being mean to other people?

Some people say it’s a lot of fun to be meaner than it is to be nice. Thought to be polite and have proper manners. My best female friend said it’s a lot more fun to be mean to best friends because we know them well and it’s easier to be brutally honest. We gotta call it as it is. Plus we brutally insult each other and we are nastier to each other. That’s how we bond. She said I should know that and stop being so sensitive. Just like easier for them to be meaner and pick on me more because I’m their best friend and they do to use their other friends but not like they do to me. They are a lot more meaner to me. Should I get new friends?