The running theme throughout the film is friendship and no matter how many times you mess up, you'll always love each other. As long as you don't mess up by cheating on them with their future husbands, then you're safe. There's nothing worse than feeling like you're all alone and even though the world can feel like it's against you, your friends are like the family that you've chosen for yourself. Which, if anything, makes them closer than family because they're hand chosen. You only tolerate your family because of blood and all that.
