My daughter doesn't have a best friend

So my daughter is a very popular girl in her class, but she says to me she doesn't have a best friend. She feels like all her friends have that one friend in particular they can go to but she feels like she hasn't. I feel like it's because she is so independent and hangs out with a lot of different girls in her class not just one, but she gets upset by this and feels like no-one wants her as a best friend. This breaks my heart. Have anyone else had this problem? She is 11.