I found that this illustration provided by Chesterton to be rather lucid, as it most certainly applies to my relationship with Mary Beth.
Perhaps the most everyday instance of this point is in the case of women; and their strange and strong loyalty. Some stupid people started the idea that because women obviously back up their own people through everything, therefore women are blind and do not see anything. They can hardly have known any women. The same women who are ready to defend their men through thick and thin are (in their personal intercourse with the man) almost morbidly lucid about the thinness of his excuses or the thickness of his head…Love is not blind; that is the last thing that it is. Love is bound; and the more it is bound the less it is blind.
My wife is my greatest advocate. She will fight you to the death if you speak a word of ill will. Yet, she is my greatest critic, a purging fire.