1. The guy likes her and she doesnt want to hurt his feelings. So she makes him her best friend as an effort to keep him from trying to sleep with her.
2. The woman and the dude are casual sex buddies.
Note: this only applies if the two are not actualy best friends.
Dude: I love you.
Chick: Awwww I love you too. You're my best friend.
Chick 2: Hey best friend. What are you doing tonight?
Dude 2: You.