Author Austen has already pointed out that a couple who brings out the worst in each other, like John and Fanny Dashwood, is a problem to themselves and those around them.

I think the point is not carried through with the Middletons, as they bring out good things in each other: Lady Middleton supplies some elegance to Sir John's constant socializing, and Sir John makes the parties jovial and pleasant, which Lady Middleton could not be imagined capable of without him.

I would suppose that Mrs. Dashwood and her late husband brought out the best in each other, since their ten years at Norland at the late owner's behest is filled with care, kindness, and domestic comfort.