First Aquinas (Summa Contra Gentiles 3.123):
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).
As with Aquinas, Hume places the emphasis on prudence, reason, and friendship ("Of Polygamy and Divorce"):
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.
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. The common link is almost certainly Cicero, although I haven't actually read anything by Cicero about marriage; that is, most of the commonalities between Hume's moral theory and Aquinas's (and there are many more than one might expect) are explainable by the influence of Cicero on Hume. Aquinas has the stronger view of the role of friendship in marriage, of course; both of them take friendship to be essentially constitutive of marriage, and Aquinas has the stronger view of marriage, so his position requires him to make stronger claims about the friendship involved.
