Blessed are You, Hashem, who gladdens the groom and bride. Until this point, they were two single individuals, who may or may not have had a prior relationship.
Marriage definitely adds a new degree of closeness in any relationship.
The answer is that before these two souls were born, they were one in Gan Eden, in paradise. When a couple marries, they are joining their souls together once agagin, albeit here on Earth as opposed to in heaven. They find their other half, the half of them which they were united with in a world of spirituality. Being that they at one time were united, of course now they will be the closest friends possible. The couple is once agagin sharing one neshama one soul, in a bond of endearing friendship.