It’s not very romantic, first of all. Did Romeo and Juliet marry their best friend? Did Liz and Dick marry their best friend? Did Scott and Zelda marry their best friend? Did Rhett and Scarlett marry their best friend?
A married person has to fill so many roles already: husband/wife, parent, sex partner, wage earner, handyman, cook, mental health professional, grammar coach, etc., etc., etc. A little help on the best friend front would be a welcome breath of fresh air.
I don’t know who my wife’s best friend is and I don’t care, as long as it’s not me. Men: if you need a best friend, buy a dog.