How is that enemies became best friends or close friends?

This makes no sense. If their enemies or make you miserable, you shouldn’t be friends. My sister said her and her best friend couldn’t stand each other and always ragged on each other. Her best friend would give her a hard time so much because she was a good girl and took the high road but my sister handed it okay. Then they became best friends. That’s weird right? I think that’s so bad.