I think when you get married, that your spouse ought to be your best friend, but while we're young people on the dating circuits, that's not necessarily the case. I mean, the vast majority of relationships we enter aren't going to be permanent...
What if you had a brother or a sister who you counted as your "best friend"? I'm not exactly in that situation, but there are a few guys in my life whom I have literally grown up with - as in I've known them since they were primary schoolers - and I count and treat them as my brothers. We literally know each other back-to-front. They're just like my best girlfriends, but they're male.
As "friends" I'd definitely be closer to these guys than a boyfriend I had only been dating for a month or two. However, I guess once you've been formally attached for a couple of years, then things might change.