She is not your best friend

Athol explains the important difference between "husband" and "best friend":

Athol:  if you asked me who I thought Jennifer’s best friend was, I’d say it was her college roomate.

Jennifer: Okay, my first instinct at that last sentence was to be insulted and upset that Athol doesn’t think he’s my best friend.  Then I thought about it for a moment and realized that he’s more than my “best friend”…I’m not sure there’s a word for it. 

Athol:  I think the word is “husband”.

A best friend is, almost invariably, of the same sex.  For the individual who thinks his spouse is his best friend, ask yourself if your spouse knows as much about your past as your best friend of the same sex.  Your best friend can't be your spouse because your best friend is the person with whom you discuss your problems with your spouse.

Your best friend is someone who always has your back, especially when you're in the wrong.  Your best friend is someone who will help you bury the body, no questions asked, and never even mention it again unless you bring it up.  Your best friend is someone who says "yeah, of course" before you even finish the question.  Your best friend is someone who will lie for you and back up your story even when he doesn't know what it is.

As a general rule, these attributes seldom apply to male-female relations.  I'm not saying that one's spouse can't be one's best friend, only that it is both unlikely and unhealthy.