Why do girls get mad when you like their best friend?

Pretty sure its not a guy thing so im gonna be talking about the girls here.<br><br>I had a girlfriend, all was going well. She had a friend who happened to be super gorgeous too. I got to know her friend and we became friends aswell. She sees us talking and she gets mad at me for being interested in her friend. I tried to explain to her we were just friends but she doesnt believe me. Next thing i know she breaks up with me. What is this?