> After all, if your spouse is your best friend, then whom do you complain to your spouse about?

I agree with you, with just one small tweak: if your spouse is your best friend, you should complain to your spouse about her. That's in fact what separates a "real friend" from an acquaintance for me - the presumption that my friend really cares about me and is willing to modify their behavior to improve my happiness, within limits.