The only way a opposite gender best friend situation can work, is if both people don't know or aren't friends with any of the other peoples friends. Don't get me wrong you can be amazing, fantastic, indescribable friends if you're in a group with other people, but knowing that you can say that someone is your best friend and that they can say it too and no one will think twice or think to themselves, 'well I'm her/his best friend too' is when everything just falls into place. Being best friends is a possessive selfish thing, you want to feel like you own that person, and feel like they own you, and same goes for them, you want to own each other as much as you want to be owned by them. You want to know that if someone asks your best friend 'who's your best friend?' that they're going to say your name straight away, regardless if you're there with them or you're on the other side of the world. That knowledge is consolation enough to live life in a perpetually happy mood.