For a relationship to really work, the people involved have to be more than just partners or lovers they have to be best friends. Best friends
Sure, romantic relationships are nice, but let's face it: you never have to ask yourself whether you know your best friend better than a boyfriend or girlfriend. Your bestie has been single right there with you, she's seen you in relationships, and you've seen her in them as well.
A girl best friend will there for you no matter what. Even if you do something horrible they will be by your side. It's someone you can trust, and be able to tell them