I've once had a gal tell me, "You're my best friend and soulmate." I was deeply touched by the sentiment, but this thought made it necessary to pull back from the deep friendship. She had a man already, and they had a kid, although both came along long into our friendship.
Song of Solomon says, "“I am my beloved’s, and his desire is for me." This is absolutely critical to a good marriage. Allowing another person other than your spouse to fill the role of best friend and soulmate, such as it is, spells disaster. When God says that in marriage, two become one, we should never attempt to break off a part of that one and attach it to a separate entity.
(Yes, I knows, says the unmarried man. wink emoticon )