The greater a friendship is, the more solid and long-lasting will it be. Now, there seems to be the greatest friendship [maxima amicitia] between husband and wife, for they are united not only in the act of fleshly union, which produces a certain gentle association even between animals, but also in the partnership of the whole range of domestic activity. Consequently, as an indication of this, man must even 'leave his father and mother' for the sake of his wife, as is said in Genesis (2:24).
Hume, Of Polygamy and Divorces:
But friendship is a calm and sedate affection, conducted by reason and cemented by habit; springing from long acquaintance and mutual obligations; without jealousies or fears, and without those feverish fits of heat and cold, which cause such an agreeable torment in the amorous passion. So sober an affection, therefore, as friendship, rather thrives under constraint, and never rises to such a height, as when any strong interest or necessity binds two persons together, and gives them some common object of pursuit. We need not, therefore, be afraid of drawing the marriage-knot, which chiefly subsists by friendship, the closest possible. The amity between the persons, where it is solid and sincere, will rather gain by it: And where it is wavering and uncertain, this is the best expedient for fixing it. How many frivolous quarrels and disgusts are there, which people of common prudence endeavour to forget, when they lie under a necessity of passing their lives together; but which would soon be inflamed into the most deadly hatred, were they pursued to the utmost, under the prospect of an easy separation?
It's interesting that Aquinas and Hume hit on roughly similar arguments in criticizing divorce; and perhaps even more interesting is the importance given to friendship in their accounts of marriage. Of course, Aquinas has the stronger view of the role of friendship in marriage; which is not surprising, since he has the stronger view of marriage.
